just leave him. he is not starving, as the doctor has already said. he will not allow himself to starve. he is getting exactly what he is craving.... attention.
many, many kids regress when a new baby comes along. your son is not the only one. kids that were once toilet trained start having accidents. kids that haven't used a bottle in months, start demanding one again. kids that have slept through the night for over a year start waking again.... it's all for the same thing.... attention.
your son is feeling like he isn't the centre of your universe any more... this is how he gets to be that centre again. he doesn't make a differentiation between "good attention" and "bad attention", to him, it's all good. it means mummy still cares.
i think that if you started ignoring the behaviour, after a few days or a week it would go away.
there is nothing physically wrong with him. he's still flourishing (if the doctor has said he's still above average, he's flourishing). you could try reverting to "baby foods" and see if he eats them.... prepare one meal of purees and then one meal of regular (at the same time)...offer him the regular, when he starts to suck, just say, 'oh, i see you want to be a baby... here, i have baby food for you...' and give him the puree...then see what happens. if he demands his regular food, then say, 'i'm only giving this food to a big boy. if you are going to act like a baby, then you will get baby food.' give him another chance with the big boy food...see what happens.
