DD1 used it as a booster for quite awhile, until we got our new to us car a couple months ago which has short buckle stalks and it was just too hard to buckle the seat belt around the wide seat.
It was a great fit in our van and a great fit for her and really comfy. Good belt fit in the car also, but just hard to use.
Converting is not fun but not too bad either.
ETA: Forgot to say, if it works well in your car, I would go ahead and use it as a booster for now and re-evaluate in a year when your son is ready for it. I liked it a lot better than the booster she is in now, but at least she can buckle up nice and easy.