Make him oblivious to all changes and sexual encounters

"Hey, turn around and let me write on your back."

"Sure, no problem," Dominic says as he obeys.

Jeremy has been wanting to put him in place for awhile. He also sees this as an opportunity to work out his attraction to him. He scribes onto his roommate's upper back:

'Always lets Jeremy write on him without complaint'

'Unaware of all changes to him and other caused by the marker'

'Oblivious to sexual acts he witnesses or is involved with'

Jeremy thinks he's done, but then realizes Dom is too straight to fuck. He quickly pulls down his pants and goes back to write just above his ass cheeks:

'Asshole is always ready to be fucked'

Just to make sure he's always clean. Jeremy quickly confirms he phrased it right by asking, "hey, answer me completely and honestly: what do you use the toilet for?"

Dominic replies, "Uh, piss? What else would I do in there? I know that you-"

"Okay, shut up now."

Dominic proceeds to do so. Jeremy is enjoying this newfound control over his roomie. He spanks him, which the other man does not react to. He then takes it further by stripping down in front of him. Dominic is entirely unfazed.

Jer asks Dom as he palms his dick to get hard, "hey, tell me honestly how my cock looks?"

"Good, I guess. Can't say I have many strong examples to compare it to. Other than mine, which I'm obviously biased towards."

Now at full mast, Jeremy thrusts his dick forward with the marker at the ready before taunting, "Well why don't you tell me how long you think it is?"

Dominic takes a hard look at the phallus, paying no mind to Jeremy writing on himself.

"I dunno, maybe over five inches, but on the shorter end."

Finished writing, the marker user stands around for a few seconds with a smarmy smirk growing on his face (along with something else on his crotch), before asking, "I'm sorry, but could you repeat that?"

Like he was in the middle of a different scene with Jeremy, Dom yells, "12 INCHES! God, you don't have to gloat about it like an asshole!!!"

Jeremy busts out laughing, both in pride and being amused by how easily his plaything was tricked by the commands he wrote. Just by writing '12" Dick' on his thigh, he must have overrode the memories Dominic just had and replaced them with memories of him bragging about a 12 inch dick. God, this marker was fun.

Dominic then yelled, "Whatever! It's not like you're going to use it before dying alone, pussy!"

This snapped Jeremy out of his joy. "God he's an asshole. I guess making him obedient doesn't stop him from being a dick unless I tell him to."

He considers taking the unawareness away from Dominic to humiliate him.... but decides against it. A part of him really likes the idea of getting back at him without him even knowing. Jeremy jabs back to him as a command, "oh yeah, well why don't you lie down on the couch, jack yourself off, and brag nonstop about everything and anything great about your dick."

He fully takes the bait and goes along with it, retorting, "okay Jer, maybe I will!"

He does as you said, laying on the sofa, tugging on his penis as he went on and on about it:

"So it's 8 inches long, with about an inch of girth. It's pretty meaty. The head is pretty big and sensitive. I've got some pretty well-kept pubes, not being too long or crazy. My balls are very plump, and chicks love to play with them. They love the musk and mouthfeel of my cock. I know how to fuck their throats in a way so they don't gag too often. The last one who sucked me off said she loved it more than her boyfriend's. I might an expert at-"

Throughout all of this, Jeremy has been bent over his face with the marker. He took his time, making sure nothing is too big or scribbly as he writes on his roommate's forehead while he rants. He brushes back some hair as he finishes off what he wrote. He then stands on the couch cushions, squats down with Dominic's head under his crotch, and takes advantage of what he wrote:

'Auto-sucks dicks perfectly when presented'

Jeremy pushes his erects cock towards Dom's lips, which then open and wait patiently for the penetration. He then slams down with no worry for the other man's well-being-he was going to suck it perfectly, after all. And suck perfectly he did. Dominic swallows the phallus with grace, gagging on it just enough to make his roommate feel good.

Jeremy gets into the swing of things, thrusting with reckless abandon. He sprawls himself over the torso, entering a sixty-nine position. But instead of servicing him in-kind, Jer just lets Dom jerk it. After all... he wasn't done using his canvas. He positions his marker, and begins writing on Dominic's pubic area as he fucks his throat without remorse. "It would be upside down when he stands up, but it should still work no matter how it's oriented."