You need a harnessed seat. Really. Your child is still a bit too young to safely use a booster. Plus, on a long trip with him sleeping, it is NOT safe. When he falls asleep, he's slouch down, flop around, not be seated correctly. It is MUCH better to get him a harnessed seat.
The Cosco Apex is a good bargain seat. Harnesses to a max of 65 pounds and then becomes a decent booster.
The bigger Britax seats are VERY comfy and safe and will allow a lot of sleeping comfort. They're expensive.
The Regent is the biggest harnessed seat available, and very comfy. He can use it with the harness until about 80 pounds. It is well padded and provides some sleeping support (not as much as the marathon or boulevard or decathlon, which are the big britax convertibles, but it's decent).
Depending on how much you are willing to spend... I would probably go with a Britax convertible. It will be the most comfortable for sleeping. But the Regent would be a better deal as far as longevity. Do you think you'll be having more kids? If so then definitely a Britax convertible (if he's not too tall).
If you're looking to not spend much money, then go with the Apex IF you have adjustable headrests and/or high seatbacks in your vehicle (a requirement of this seat) as it doesn't cost much more than a really good booster and it will allow him to be harnessed for much longer, which is a huge safety measure, especially when sleeping. It has an additional foam insert which should be enough padding. It's also a pretty wide seat, so if he falls asleep, you can give him a small pillow to tuck against the SIDE of the seat (not the back) to rest his head on. Just make sure it doesn't interfere with the harness at all.