Yep- a high back booster or harnessed seat would work great! We have a 97 Wrangler and have never had a problem getting car seats to work well, except a few RFing seats, but with a 5 year old you don't have to worry about that. Unless he is VERY large for his age (at least over 80 lb, and over 4'9" tall) he should be in a booster of some type even if there was head support. If he is under 40 lb or doesn't sit right in a seatbelt/booster, then he should be in a harnessed seat. For a booster look at the Graco Turbobooster, Britax Parkway (will last the longest, probably, which is good since he'll always need the head support) Recaro Boosters, Compass Booster, or really any other high-back booster except the Dorel (Cosco/Safety First/EddieBauer) seats. Dorel requires support behind their seats, which the Wrangler doesn't have. The most economical is probably the turbo booster.
If you want something even safer or need a harnessed seat I'd look at the Regent or Radian. We have a Radian and it works great in the Jeep.
Hope that helps.