How do you reinforce that you are boss? Sadly we played and loved on ours way to much, and now he is becoming a bit aggressive, to the point where I won't go into his pen anymore

I am hoping this doesn't pass on to his ram lambs?
It will if you don't learn from your mistakes.

Some people say after the ram has entered this phase of his maturity and has already wrongly imprinted on humans, the only recourse is to kill him, but I'd say it's worth a session or two of training to see if you can change his mindset. Never hurts to try.
Was his sire aggressive? That's valuable to know before buying a ram lamb, as that can definitely be passed down if they are just naturally aggressive.
Reinforcing you are boss does not include hitting him in the head or face when he's butting....that's what he likes and he sees that as a further trigger to challenge you. You may need someone bigger/stronger than you if he's a big boy....or not, depending on how strong and determined you are and his relative size to you.
If a ram lamb it's relatively easy....you just tackle him at the first sign of ram behavior....backing up and lowering head, butting...even lightly, etc. Bend his head back to one side and flip him onto the ground and hold him down with all your wt. on him, even if you have to lay on him or straddle him....and keep him there for awhile until he stills and is no longer fighting and struggling against you. They say it helps to also yell loudly while doing this and terrify the crap and all out of him....I'm more of a silent killer, so I throw all my muscle and wt. into it and then growl into his ear, holding onto his throat tightly as he lays on the ground.
At that point he's getting a clear picture that we are a predator, not another sheep. When he's perfectly still and I feel like he's got the message I let him up. As he's getting up I'll grab his loin/flank area with hands formed into claws and goose him along as he runs off. Depending on the age of the ram or natural demeanor, you may have to repeat it. As I deal with Katahdin rams, they are naturally docile so it doesn't take much to get things in their heads. Funny thing is, they also don't seem to be too scared of me to approach me for feed or such after that, but they definitely never try the butting thing again. I've found Kat sheep are largely ruled by their guts.
You might try it and gauge the results.....one never knows until they try.