Marisa has been in a booster since she has been two, with a harness. We were also worried about her taking off the straps, but we encountered the opposite - she was so excited sitting in her new seat that she doesn't move.

She is on the passenger side of the back seat, boosters can be placed anywhere. They have special clips to hold the seatbelts low and together - not long ago we actually had the child restraint people check and help us fit it to Marisa's needs, when the capsule was put in for Elijah.

So I would say, give it a try, just leave the baby seat in just in case. Although I would assume that baby seats are designed to cater up to certain sizes / weights so you might have to check that out too, and see which is the safest option. If he does sit in his booster seat without trying to get out of it, give him lots of praise and reward his good behaviour rather than bad.

Perhaps some distraction might work - does he have many toys in the back? I find Marisa is much easier to manage in the back when I give her different things top play with in the back, although she often will be fine in the back seat. It's just when she's tired or bored she's more trying!