This is something that only you can decide. It sounds like he does have some growing room, weight-wise, for rear facing, though you’ll also want to double-check that he has adequate room above his head (at least 1” of seat shell). Legally no states require rf past the 2nd birthday so if you choose to turn him now you can do so. The AAP and other safety organizations have long recommended rf to the limits of the seat, but we don’t have studiess telling us how much safer it is. Is it safer? Yes, but the most important thing you can do is use the seat correctly, regardless of what direction it faces.
Buying a new seat is certainly an option, and based on him having only a small amount of growing room even for ff, you should know that time is coming sooner rather than later. If you buy a larger convertible you can contimue to use it rf for another year or more and then ff until he is booster ready in a few years. Alternately if you are comfortable ff him, your next seat could be a ff only seat that is used with a harness now and a belt-positioning booster later.
Personally if this were my situation I’d buy another convertible and keep him rf for at least another year, but again, this is a decision for you to make.