You only have one option :-) Your child will need a larger 5 point harness for a few more years. Once a child is OLD enough and mature enough, the next step after a 5 point harness is a high back belt positioning booster, but the maturity to use those correctly comes between 5 and 6 years old.
The Safety First Air Protect only goes to 40 lbs. Do you perhaps have a Safety First Complete Air, one of the original models? Those had a 50 lb weight limit. Let's figure out what you do have because if your child is actually in an Air Protect, we need to get him into a new seat today.
Meanwhile, the cheapest seat that would be appropriate for your child would be the Evenflo Secure Kid 300 or the Evenflo Secure Kid 400 or the Graco Nautilus. You need a harness that goes over 50 lbs, a taller harness that accommodates an older child (don't pay attention to stated height limits, they are often inflated ridiculously and will not actually fit a taller child), and a seat that converts to a GOOD belt positioning booster for later use. (There are many seats that convert to BAD belt positioning boosters and you don't want those either.
A word of caution: there are several seats that might sound good at a slightly cheaper price point. Don't get them. An example would be the Evenflo Generations 65. Despite claiming to harness kids to 65 pounds, it has a shorter top harness position than your current seat, if it's the Complete Air, and despite claiming to booster to 100 lbs, not only is it too short to last as a booster, it makes a bad and dangerous one due to poor belt fit.
What car is the restraint going into?