Keeping your son in a harness for as long as possible is safest, you are correct on that :)
The minimum age a child should be in a booster is 4 with a minimum weight of 40 lbs. Children under 5 are at significantly higher risk of death or serious injury in an accident if they have been moved to a booster seat. (this is safest practice, not what it says on the booster seat box). So you are right to be looking for a 5 pt seat for your child. And then remember that kids should be in High Back booster seats for as long as they fit in them as they are significantly safer than the backless boosters. Backless boosters are truly meant for much older kids.
It really depends on his height and weight but 2 seats that are great are the Graco Nautilus and the Britax Frontier. If you have a headrest in your car where the seat will be the Safety First Apex also goes to 65 lbs, but it's not as good as the other two. I have a Frontier for my 4 year old.