If they are still hockey players then the boyfriend is someone unaffiliated with hockey and Shane has been dating him probably since before he met Ilya.
Boyfriend is out and Shane is not (for obvious reasons) and while this doesn't necessarily make for an unhealthy relationship Boyfriend is, as established, an asshole. He's not shy about reminding Shane of how lucky he is that he puts up with the secrecy and how impossible it would be for Shane to find someone else in his situation.
They've also been together since they were teenagers so Boyfriend is the only guy Shane has ever been with and the only one who knows Shane is gay, which also means that Shane has no one to talk to about any misgivings he might have about the relationship.
The story is the same as in canon from Ilya's perspective until the CCM commercial when he hits on Shane in the shower and Shane, visibly into Ilya and fighting with himself internally, tells him he can't because he has a boyfriend. Then he panics about what he just told Ilya but Ilya promises not to tell anyone - they have the same secret, after all. Sure he's disappointed but he can always find other people to fuck (that hollow feeling in his chest feels like a lot more than just disappointment but Ilya ignores that).
Every time Shane and Ilya catch a moment alone over the next couple of years, Ilya asks Shane if he's still with the boyfriend and the answer is always yes. Around the third time, Shane is feeling vulnerable because they just had a fight the week before and Boyfriend has been ignoring him and he doesn't have anyone to talk to about how awful he feels, and he just kind of. Starts telling Ilya about it.
And Ilya listens and then tells Shane, "He sounds like an asshole."
Shane feels conflicted about this comment because on the one hand, it kind of confirms what he's been thinking, but on the other Ilya doesn't actually know Boyfriend and all he has is information from a very biased source.
"He's not," Shane finally says. "We just got into an argument. Couples fight."
Ilya wants very much to disagree, because Hollander sounds fucking miserable talking about this guy, but he wisely concludes that this would just make Shane dig his heels in further and refrains.
Despite Shane's protests, every time Ilya asks him about his boyfriend after that he looks more tired. This guy also clearly makes him insecure as hell and Ilya hates seeing the effect he has, making Shane fucking Hollander carry himself like he's two inches tall and trying to disappear.
Ilya is itching to tell Shane to dump Boyfriend, to find someone who'll treat him like he deserves. Whenever he indirectly broaches the topic though, Shane gets defensive, and Ilya doesn't want to push it because as much as he'd like to get Shane away from this guy (and into his own bed) he also doesn't want Shane to shut him out and isolate himself even further.
Then comes the day when Shane actually approaches Ilya first. He looks nervous but determined and at first, Ilya thinks this is it. He finally remembered that he's Shane fucking Hollander and kicked that guy to the curb.
What Shane tells him isn't quite that. Ilya still perks up, because he cannot believe that Boyfriend, this absolute piece of shit moron, suggested opening the relationship and delivered Shane to Ilya on a silver platter.
Because Shane's first instinct, upon getting permission to fuck other people, was to seek Ilya out. He looks cautious but hopeful when he asks if Ilya's still interested in hooking up, like the answer isn't obviously yes.
Yes, Ilya's still interested. But not just in hooking up. Not in being the side piece.
He's gonna seduce Shane right from underneath Boyfriend's stupid, ugly nose and treat him with the reverence and care he actually deserves.