Sorry mom, he's not big enough for a booster seat with the adult belt. Regardless of what 'the box says', he's not big enough for an adult belted booster seat.
The Graco Nautilus is $150, will harness to 65lbs, then becomes a high back booster to 100lbs, and eventually a no back booster. It will be the last seat he needs ,and will last him with the harness until he's at least 7yrs old, and then becomes a good booster seat.
If thats too much, the Evenflo Maestro will harness to 50lbs, then becomes a decent booster seat. It fits most kidsi n the harness to age 6, and it boosters to 100lbs. Its $70.
The Evenflo Generations 65 will harness to 65lbs and fits most kids to age 6/7 in the harness, and then boosters, and its a good booster seat. This is $99, and its a GREAT choice.
Please don't put your little guy in a booster seat. These seats listed above will last him until he no longer needs a car seat or booster seat, and they are all well worth the price to keep him harnessed for a few more years, quite a few more pounds, and alot more inches, all of which he needs to safely ride in a booster seat.
For what its worth, my 4.5yr old is 43" tall and 45lbs, and he's still harnessede in a Britax Regent. You can find these on clearance right now for $150 or so, they have discontinued that style of seat in favor of the Britax Frontier. My oldest used the Regent until he was 7.5yrs old and still had room to grow at 52" tall and 60lbs. Its a great seat too, and I would never put a child under age 6 in a booster full time. A child needs physical and mental maturity to ride properly and safely in a booster seat, so please purchase one of the seats I listed. Try them out, see which you like best, and keep your little guy in a harness!