I get crazy twitches when I am stressed out. During a particular rough patch a few years back, I had one it my eye and one in my leg that did not go away for 2 months!
Based on just his circumstances alone though, the best thing you can do for this boy is to get him into therapy ASAP. There is no telling what kinds of things he has seen and experienced at this young age! And then for his mother to not even visit, that has to be heartbreaking and confusing for him. He might not even know how to express those feelings in words yet. He is after all only 7 and this recent change in his life is HUGE and the feelings that would come from a situation like this are going to be very complex. He needs consistant therapy so he can work out some of his feelings.
I am so very happy to hear he is living with you now. He will need a lot of time to start to feel normal about his new situation. He needs to feel wanted and worthy and loved. He might need a whole lot of rest to really get his body caught up. It might take him a while to really open up to you. Sounds like you already know, but be patient with him :)
Sending you both big hugs!!!