M.R.
A Nueropsychological evaluation with a psychiatric consult is one option, but you would need a speech evaluation and an Occupational therapy evaluation also, at the very least, and then you will have to guess how these two areas interface with the neuropsychological. I would suggest a Developmental Pediatrician. You can get all these things, and everything else he needs, all rolled into one evaluation, so you do not have to guess about anything and you know that you will not miss something. Call the nearest children's hospital and make an appointment. Many will likely tell you that the birth of your second is the cause, but I think that is only a complication to what is going on here.
M.