He's behaving totally normally, given his living conditions.
Move out and encourage her to focus on raising her son.
as a woman whose mother married a man i HATED (and later beat me), i HIGHLY suggest you work out these issues with the boy 1st.....just "marrying her", will only make that child hate you more.....grandma is obviously contributing to this issue, and mom doesnt seem to mind.....
at your's and the mothers age(unknown), you don't have to be married to show respect....yes it's possible mom's had other boyfriends....maybe he thinks you're "too old"....in that case, prove him wrong! if he thinks you're not "good enough for mom" prove him wrong there....if you spend time with BOTH of them, not just mom, like dinner/movies etc, have "family dates" in time he'll be more comfortable....it's not good she lays with him....that affects them both, as she loses sleep, and he's too old for that.....
but please, DO NOT run off and marry her, b/c if she later decides that all the attention her son needs, outweighs you(i mean the "over" attention), then you could find yourself divorced.....seek counseling BEFORE marrying her, all 3 of you need to come to terms, esp the son.....where is the child's father? is there some possibility that the father has bearing on his attitude towards you? my stepdaughter(now 7, then 5) slept with us her 1st weekend at our house, b/c we'd literally just moved in....after that, she slept in her own room, and did VERY well, considering she and her mom live at granparents, and share a room(bunkbeds)....but we were able to break that habit as soon as we showed her how "awesome it was to have her own room and have privacy"(well, age appropriate kind)....and she loved it....i think at one point she made the point to her mom "sleep on the couch i need my space"(rude, but funny and very true)....
Boundaries is the key word here.
You have moved in and stepped right over his boundaries and you can expect him to react poorly to your presence. With time and careful management you will be fine, but it will be softly, softly for a while. It is normal for a child who has lived alone with his mother to behave in a clingy way when you are around. As far as he knows, you ARE stealing his mother from him. You are certainly stealing his mothers time and attention from him.
Clear rules will have to be put in place after discussions between you and his mother. Him sleeping with you is an absolute non starter. The marital bed should be a no go zone if you wish to maintain your relationship with his mother (I suspect he knows this and is testing just how far he can go in stepping over YOUR boundaries)
He is too old to need his mother to lay with him every night to get him to sleep. That is pure manipulation and attention seeking behaviour on his part. Make sure mum spends some time alone with him each evening but when bedtime comes, he settles to sleep himself after she has read him a few pages of his favourite book, or listened to him reading himself, if that is more appropriate.
The issue of him running off to his grans has to stop too. It is reasonable, again after discussion between the adults, to decide how often he can stay at his grans, and which days are suitable for this. Clearly on a school night, he should be home in his own bed, unless otherwise arranged, otherwise his school studies will suffer. If he runs away to grandma, she needs to firmly tell him that it isn't his day and bring him home, otherwise no issues will ever be resolved at home. If this isn't nipped in the bud it will get worse as he gets to his teenage years. Grandma may have a vested interest in letting him stay but it is undermining the rules of you and your partners home if she lets him stay after you have discussed this with her and explained why you need to have the rule.
At the end of the day, you have moved in and have no defined role in his life. You are his mothers boyfriend (and for all you know, he may have had many temporary 'dads' over the years-how is he to know you are going to be permanent?) RockRose may have been clumsy in the way she said it, but she has a good point. If you love this woman enough to want to share her life, if you have been nothing but supportive as you say you have, if you truly want to be part of this kids life as well as his mothers, then maybe you should consider marrying her and showing them both some respect.
You're 53 years old, for God's sake. You're older than I am. Marry her or don't come into her home and live with her.
You are correct he needs to have some boundarys, his own Room to sleep in and she should not lay down with him,he will like his independance an space.Granma should do the same it isnt him its the adults here allowing it. He needs you and a lot of guy activitys and sports in his life Wear him outwith Activity you and he are the guys of the House, let the girls do a few girl things. He needs you.