Have you made note of any triggers? When I get stressed beyond my threshold, I'm prone to biting my arm really hard...if I have no way to escape from the stress.
If it happens, usually it happens if I end up in an argument with a family member and they are attacking me on an emotional level an putting me through lots of stress. Thankfully this doesn't happen often, but it is frustrating for both of us when it does. I don't like to hurt myself, but when I feel extrememly stressed it is a knee-jerk reaction... The pain does at least two things (besides giving an embarrassing bruise). First off it delfects some of that extremem anxety into physical pain. While I'm hurting it distracts me from the emotional pain.
2ndly if I do it in front of the person who is causing the stress, it hopefully shuts them up and makes them stop. That way we can both calm down... If that doesn't work, and the person continues to go on then it intensifies the uncomfortable feeling.
The best I can describe is likely the feeling a wild animal gets when it is suddenly cornered and trhown into a small cage. Without any escape it will likely bang up against the bars hoping to find an escape.
As a child I would also do it sometimes out of intense boredom, and mostly when I was upset and crying over something., It was likely an attempt to stop the emotional stress I felt.
Now days I'm trying to cope better and find alternatives to coping with the stress. I think I've made great progress, but sometimes in an outburst.... I'm trying not to let myself get stressed to the point I get those outbursts, but it is an ongoing process.
MJI, who knows this subject the best, is absolutely correct about the reaction to stress. Also for any kids often negative attention is just as rewarding as positive attention.
Another factor to consider is the non-verbal status. Have you started introducing a few signs? If a child has no to express pain, anger, frustration, etc, they use what they do know - if I hit, mom comes. The "ow" is self-talk, perhaps trying to decide what to do , or perhaps trying to tell himself NOT to do it. Self-talk is not unusual for any child. My son is typical, yet when he was young he used to tell himself not to touch the electrical outlets every time one caught his attention.
Try learning a few basic, necessary signs and introduce them (along with the spoken word) so that he has a way to tell you he's hungry, thirsty, sick, etc.
This (signs) also got me thinking about PECS. They need not be elaborate, just Polaroid photos with Velcro on the back on the fridge (oops, Polaroid went under, *sigh*).
I was smacked and overly well not mega battered don't miss judge me here, my step dad didn't kmow I was autistic mum step dad and biological dad all knew I had a birth issue I didn't I was born deprived of oxygen my dad doesn't mince wrods he said I came out black/blue so acute toned on my skin he litterally no lie said to the nurse attending (midwife) Doctor etc (is this my sone he's so dark skin toned?) They put me on a ventilator, in fact I was not held by my mum or dad until 3-5 weeks after my birth, and was on an automatic ventilation machine.
Now mum and my bioliogical dad were great, yeah my biological dad allways was there paid maintenance you know, allways paid direct postal orders me and sis to but paid mum the minimum maxed amount, yeah sounds low but get this he allways sent seperate birthday cards and even weekly comics in recorded post even 10 gbp for easter in postal orders and comics star wars toys etc, he gave me and my sister stuff and money had we accumulated those payments like we should have right in banks the amount would have way way exceeding the low end, so yeah dad maybe got bad rap from mum and me step dad (still dad to me), but all 3 both dads and mum saw I strugglerd to learn, this caused big issues in that then I'd smoke weed and be a divvy and my step dad I used to think resented me but nah he loved me I loved him equally still does and I still do, him and mum specially step dad saw a big issue in learning, they asked the school well .gov for help, they didn't then have child autism understanding like today, so both schools primary and secondary overlooked me said naughty kid, so my step dad and mum thought yeah we'll trust them, and you know yeah I flopped at school I smoked weed 15yrs old made my memory majorly worse, ruined family relations, mum and step dad trusted the child physchiatrists assigned by .gov they said he's naughty not autistic etc, they reluctantly accepted got to be right, in the end they wrongly labelled me disruptive, naughty and easily led, lol In 2010 years on, yeah weed and autism and dyspraxia dont blend caused me and step dad to go full fight I got booted went a hostel and mates got me in there shared homes to eventually my rented flat, i forgave me step dad he said educated guy big time that me and your mum allways suspected aspergers maybe and or dyspraxia but the .gov child physchiatrists said naughty kid, he said if you get diagnosed I'll hope you can forgive me because I'd never ever touch you knowing you were autistic me and your mum argued but they said nah naughty kids so yeah I slapped you told you off!
So you can see I forgave him it was a miss diagnosis at primary through secondary man I should sue them, get the right care and support worker for your kid start diagnosis go from there.
But yeah I never got told I love you enough, or given a fair sot, my advice praise your kid and encourage his/her abilities and hobbies. that's what messed me up school wise.