Welcome to the boards! Hopefully we can answer some questions and give some ideas here.
Forward-facing kids often have head slump. It happens. Unless he has a medical condition it's not a safety issue, only comfort (and then only for some kids- some don't seem to mind at all.)
You should have the vehicle seat upright when installing a car seat, so no, don't recline it. The older Boulevards placed a weight restriction on forward-facing recline so depending what model you have you may need to check that.
So basically, there's not much you can do to change the recline of the forward-facing seat. What you can do is try some interventions to see if they increase his comfort. First, make sure he's harnessed snugly enough, using the
pinch test. Once that's checked, try some positioning ideas. One is trying a child "neck pillow" BACKWARDS- don't place it behind his neck, but in front instead. One is to, similarly, use a very small pillow (or even folded piece of cloth) next to his head when he starts to fall asleep, to "wedge" it in; this can be something he does himself when he gets sleepy or something you pull over to add when he falls asleep and slumps forward. Alternatively to using physical props, you can teach him to look up when he gets sleepy, as looking up at the ceiling while asleep will keep the head back. This often helps children stay in position in a harnessed seat, and is a valuable skill to learn as that will make sure they can later stay upright in a booster and seatbelt only when they get to those stages. Good luck!