Devils advocate here too 👋🏻
I have lots of male friends, one in particular I would say we’ve been through some heavy shit together and known each other for about 17 years. Never been tempted romantically, despite being single at the same time for long periods. I say “I love you” to all of my best female friends, but this one particular male friend I do too. I do love him. He’s been there for me at some awful times, and I’ve supported him lots of times too.
If he’s going to travel or has a big event, I tell him I love him. When he’s struggling, questioning himself, feeling miserable, or generally going through something, I tell him I love him. I think I feel a need to tell him a lot, not just because I do love him, but also because he’s an only child and his parents are both dead. I feel like he needs to be shown love and to feel cared for. His wife loves him, of course, and his child loves him and they no doubt tell him they love him plenty. He is loved by his male friends, I’m sure. I doubt they express it much, though, so I make sure that I make the effort to share my feelings with him when appropriate/needed.
My husband knows I love my male friend. Doesn’t bat an eyelid. His female friend was going for a big op and was upset and tearful about the GA and the whole thing. He told her he loved her before he came off the phone. He offered practical help, and said he would wait to hear. That to me is very much what I expect people to do for their friends.
How deep the friendship in the OP’s case goes, none of us can know, therefore the question of it being inappropriate is really hard to answer. Instinctively I feel like it’s fine, but without a massive amount of context none of us will be able to definitively answer.