Hi there.
Using a pool noodle or towel is for rear facing only. You cannot use it forward facing. It can make the seat perform inadequately in a crash, resulting in injury or death.
Do you have the seat in the appropriate forward facing recline, and you've got the harness so that it passes the pinch test at the collarbone and at the hips? You've got the tether attached?
Generally, forward facing seats are more upright and cause more slump than rear facing seats. That's just due to their shape and the way they sit in the car. If you or he is very bothered by it, if he's under 40 pounds, 40 inches, and has more than one inch of headrest above his head in one of the bottom two headrest positions, you could turn him rear facing. That would help the slump, and make him safer as an added bonus. But you'd need to get the recline line parallel to the ground, so it may not fit front to back in your vehicle.
Wendy