I'm not sure. Has he picked up any new skills lately? Maybe he is concentrating on the new one and temporarily giving up old ones.
Has anything traumatic happened to him? Like divorce, any loss, or molestation?
As far as autism, does he hand flap, walk on toes, stare into space, stare at spinning objects, play unusually with toys?
I asked the same question a week ago (no answers as yet tho)...my stepsom is the same...he has regressed in evrything tho, not just with his speech but his eating, dressing, even movements. A once happy playful boy has become for lack of a better term a couch potato, just sits all day staring at the tv whether it is on or not. We were planning on taking him to a Dr but the mother has kicked up a stink saying there is nothing wrong with him and for us to stop saying something was. I hope you get the answers you need...I'm still at a lose.