So at this point I think we need to look more into the specifics rather than just abstractly discussing split attraction ROs in general.
So I followed this to your WiP thread and tumblr, which provide some pretty significant context… it’s so much easier to discuss this with those in mind.
For one thing, I guess this is a historical setting, albeit an alternate history, where it sounds like we wouldn’t be hearing the terminology like “gay” or “panromantic homosexual” or “queer.” All important because it’ll change the way they talk about the experiences. Would make it rather harder, to be honest, given he wouldn’t be able to be as precise about it.
Most importantly, I read the excerpt showing one way the conversation could end up going. I think this is what we all need to look at to get an idea for how his orientation is described and how the relationship would be portrayed.
And… at least as for me, it reads more like agreeing to a V rather than an all-around triad. It sounds like Tommy saying he likes you and is okay with having you be with Seán rather than saying that he’s entering the relationship with you as well. The romantic side isn’t all that clear to me; you can like cozying up with someone totally platonically, and I don’t see anything that indicated a romantic connection with the main character.
I also took a look at the original ask and followup and… okay, yeah, that followup is extremely out of line and invalidating to the way plenty of real people actually are. That’s never okay. However, as for the initial ask, it does make a good point about a vee being a valid possibility.
Like, polyamory in general is quite underrepresented, but when it is represented, a lot of times there’s more focus on triads and the like rather than other configurations (aside from “traditional” polygamy, which is a very different matter altogether, not at all the same sort of relationship). But it’s my understanding that a lot of times poly people who are in a relationship with each other will date separately because they’re compatible with and interested in different people, and that that can be more healthy than trying to force a link between all involved parties. I think it might be worth thinking about at least the possibility of a V in which the main character and Tommy are friendly together but not necessarily linked. Potentially involving cozying up too, sure 
Or at least making some boundaries very clear so it’s clear the MC isn’t pushing them.
Well, also be prepared for gay men to have very different opinions 
I mean, in this thread alone, we’ve been saying rather different things.