Even better is to put the medication in your carry on luggage so they can see it easily if they want to question.
Nappies, take enough for the trip over and until you can get to a supermarket. They sell Huggies over there so you don't have to worry about the bulkyness of them. If he is still on a formula of some sort, see if they sell it over there, then just take a few packets until you can get a whole can.
If you are looking at going get your passports now if you can afford it. Sometime there can be delays and the last thing you need is the worry about are they going to make it before you leave.
See if your friend can borrow a pushchair over there from a friend of hers. That way you don't have to worry about taking on over.