A.L.
If you didn't buy your ds a seat then when you get to your gate ask if there are any available seats... say 3 in a row or whatever you need. We did that when dd was 6 mos. and it helped to let her sit on her own a bit, lay down, etc. without having to pay for a ticket! Bring lots of snacks for him, a pacifier or something to suck on while taking off and landing, and books,toys, etc. that will occupy him. If he likes music you could bring a radio/CD player/IPOD with headphones that he could listen to. It'll be fine!! :)