Hi Alex
Neither your carseat or your stroller count as luggage - we have checked both, and have also used the carseat on board - they don't weigh them or count them at all.
Your stroller can be taken right up to the gate of the plane, where they will ask you to fold it, then they will put it in the hold. At the other end, it is usually returned to you at the gate, although occassionally we have had it come off the carousel (but never in the USA -always at the gate there)
If you are paying for a seat for your son, then he will also get a baggage allowance.
One important question - you are flying with Airtours right? Have you confirmed which brand of car seat they allow on their flights?
I had a LOT of bother with this when we flew Airtours last year. The agent at Newcastle was adamant that they would only allow a Britax car seat on board, and there are only a couple of styles of Britax that will fit. We had quite a fight and in the end we had to check the carseat.
Strangely, the American agent on the return journey was more than happy for us to use the carseat, in fact she suggested it. (It was a Klippan).
I would ask for something from Airtours in writing, if possible, because I had checked with Airtours 3 times, and was told that it had to be forward facing and less than 15in wide, which my Klippan was - but they still wouldn't allow it on. I was told it wouldn't fit- which was odd since it worked a treat on the return journey.
I wouldn't worry about luggage allowance at all - with 3 tickets, you can check 6 bags, a stroller and a carseat, plus carry on 3 bags, plus briefcase, purse, baby bag - so you should be well within the luggage limits.
Bev