mate i've been playing for 13 years. my advice is to go to a music store, eg Allans music in melbourne, and get your son to have a play of some. Some trumpets are heavier than others, eg mine to assist with jazz and blues style playing. its is really a personal preference to the instrument. I know when i picked up my james morrison signature i loved it, could play the full range without a hassle, but i picked up another (can't remember the name of it for the life of me) and had a hard time with the top register.
Really he is going to want/need a Bb trumpet as a C is more for ochestral playing. Something with 1st and 3rd valve slides will be and advantage in the future. Also silvver plate is easier to take care of than the gold lacquer. Student trumpets are great no need to spend the big bucks if he's not going to hang with it. Yamaha are your best bet.
I'd stick with something along these
http://www.allansmusic.com.au/Produc...7E182&p=120147
http://www.allansmusic.com.au/Produc...7E182&p=103537 <---- would be a descent starting trumpet without breaking the bank
http://www.brassnwoodwind.com.au/YamTptYTR1335.html <--- my only concern with this is it only has 1 water key
http://www.brassnwoodwind.com.au/JTR410.html <--- this was my first trumpet.
Also consider getting a 5c mouthpeice to go along with the 7c if your son has or is going to need braces. The 10c is a slightly larger mouth peice with a deeper bore. It helped me when i had braces to get a clear tone without cutting my upper lip.
in relation to secondhand instruments, alot of music stores have secondhand ones. Also the local university music department may be able to point you in the right direction
feel free to PM me with any questions mate, happy to help