If at all possible, you want to keep him in his harness. He will be safest that way. There are a couple of seats that will let you do that... Britax and Rocaro are two, but they are pretty expensive. Safety 1st has one that you can get online for about $120...and it will allow you to keep them in the harness until 65 lbs...my very large 2 year old is in one, as is my 4 1/2 year old...and then converts to a booster. This seat has one extra slot for the shoulder straps, allowing you to keep them in the harness longer. Technically, with the law the way it is, they are supposed to be in a child restraint until 4 years, but the new booster law will keep them in a booster until 8 yrs and 4'8". A pain, I know, but a five point harness is the safest...
I know Michigan law is pretty ambiguous (only requirement is child restraint until 4 yrs, doesn't even say which way they have to face, or a weight!!!), but child safety seats are my passion...can you tell I work with a lot of patients who have been in car accidents??? :)