You're right, he is NOT ready for a booster. Most kids aren't until they are about 5! There are a number or car seats with harnesses that hold kids over 40 lbs. The Britax Marathon, Decathalon, and Boulevard hold up to 65 lbs in a harness. The Britax Regent holds up to 80 lbs. The Safety First Apex holds up to 65 lbs but you need headrests in your car to use it. The Radian 65 holds up to 65 lbs and the Radian 80 holds up to 80lbs. The Evenflo Triumph ADVANCE holds up to 50 lbs. Since you son is big for his age I would definitely go with the Britax Regent. It will last you until he is ready for a booster, and longer than that if you choose. I have both the Britax Regent and Radian 80. The Radian 80 works well for my skinny tall daughter but bigger kids seem squished in it. I LOVE the Britax Regent and highly recommend it. It has a bit of sticker shock...but you can order online with free shipping and with so many years of use it's well worth the money. I really recommend going to www.car-seat.org and posting on their free message boards. Car seat safety technicians will answer your questions, tell you what seats will work best in your car, and can point you in the direction of the best deals. Good luck! It's always good to follow mommy instinct. Getting out of a booster is so easy and a two year old just doesn't understand how dangerous it is. The fact is, everyone is safer in a 5-pt harness, that's why race car drivers use them.
To the other posters...I'm not calling anyone out, but since this is a safety issue I think I need to say something.
I had the Eddie Bauer combo seat as well and the harness only goes to 40 lbs, NOT 90. You can use it as a booster from 41-90 lbs but the harness can only hold 40 lbs. Using it past 40 lbs can cause it to fail in an accident. I used to think this too, but if you read the fine print it will say to only use the harness to 40 lbs.
There are a number of car seats that hold over 40 lbs, www.car-seat.org is a great place for information.