After the fall of Nicolae Ceaușescu in Romania, it was discovered that there were terrifying orphanages full of kids who had had almost no human contact for years and were in bad shape as a result. My aunt volunteered to go to Romania to help save as many of these kids as possible.
The relevant news for you is that kids who were only three at the time rescuers arrived almost all recovered without lasting harm. The thing that saved them was LOTS of daily human interaction.
Talk to that boy as much as you can. Hold him, sing to him (singing skill is optional), play with his hands, play "piggies" with his toes, work on naming parts of his face, play "head, shoulders, knees, and toes," play with blocks, talk about colors, have one of you make a funny face and the other one has to copy it then switch roles, and so on. (Don't read too much into it if he's really bad at some of these games. My sons had no developmental problems, and yet I could not BELIEVE how badly they did on some simple mental games: "See this green card, Buddy? It's green, isn't it?" "Yes." "What color is it?" "I don't know, Dada." "This color is green, okay?" "Okay." "So, what color is this GREEN card?" "I don't know, Dada." Yes, believe it or not, this is totally normal.)
Be a little careful not to overstimulate him with too much noise, wild motion, etc., because that can be hard for him to process until he's older, but lots of calm, happy, silly, giggly talking and playing should (slowly) bring him back to the path he would have been on without the early lack of attention.
Also, as others have suggested, do involve doctors in this to the extent you can. There might be more to the story than lack of attention, and "many eyes" are better for debugging problems early, right?
The relevant news for you is that kids who were only three at the time rescuers arrived almost all recovered without lasting harm. The thing that saved them was LOTS of daily human interaction.
Talk to that boy as much as you can. Hold him, sing to him (singing skill is optional), play with his hands, play "piggies" with his toes, work on naming parts of his face, play "head, shoulders, knees, and toes," play with blocks, talk about colors, have one of you make a funny face and the other one has to copy it then switch roles, and so on. (Don't read too much into it if he's really bad at some of these games. My sons had no developmental problems, and yet I could not BELIEVE how badly they did on some simple mental games: "See this green card, Buddy? It's green, isn't it?" "Yes." "What color is it?" "I don't know, Dada." "This color is green, okay?" "Okay." "So, what color is this GREEN card?" "I don't know, Dada." Yes, believe it or not, this is totally normal.)
Be a little careful not to overstimulate him with too much noise, wild motion, etc., because that can be hard for him to process until he's older, but lots of calm, happy, silly, giggly talking and playing should (slowly) bring him back to the path he would have been on without the early lack of attention.
Also, as others have suggested, do involve doctors in this to the extent you can. There might be more to the story than lack of attention, and "many eyes" are better for debugging problems early, right?