Children should NOT ride in a booster seat until they are AT LEAST 4yrs old AND 40lbs. No back booster seats are for BIG kids, not 3yr olds, and the bare minimum on them is 4yrs old anyway. No back booster seats are the last seat a child should use, and should not be used until at least age 6. They offer ZERO side impact protection, and you see how his body slumps when he sleeps? In a crash it will be thrown to the side even farther and violently.
At his age and size, he should still be in a 5pt harness. There are many on the market right now that will fit his height and weight. Evenflo Generations 65 will harness him to 65lbs, fits most kids to age 6/7 in the harness, and is $99. It also becomes a high back booster seat. The Graco Nautilus is $150 and will harness most kids to age 7, and then becomes a great high back booster seat, and then a no back booster, to 100lbs. The Britax Regent is a great seat, we have one, and it will harness to 80lbs and most kids can easily use this seat until they are big enough for the adult belt alone (age 8, 4'9", 80lbs), and you can find this seat online for $150 or less. Its been discontinued in favor of a booster/harness combo seat.
If you insist on a booster seat for him, please please get him out of the no back booster seat. Again it offers no side impact protection and in a crash a child his age needs that. Regardless of his height and weight, his body is still that of a 4yr olds, and not mature enough for that type of restraint. A high back booster is second best at his age to a harness, but if you insist on a booster, get a high back.
A Graco Turbobooster fits small children well, and its around $50. Britax makes a booster ($120) with an anti submarine buckle between the legs that not only holds the lap belt down onto hte hips where it belongs, but it keeps children from sliding under the lap belt in a crash. We have a Sunshine Kids Monterey ($150) for our almost 8yr old, and we love it.
Again, he should really be in a harness, but the very least he should be using a high back booster seat. Please google SEATBELT SYNDROME, KYLE DAVID MILLER, BELLE'S GIFT, and EXTENDED HARNESSING for more information.