To be honest, I think he might be gay. Ive come across many stories of situations in which some male person pulled a very quick 180 and began to almost immediately stop hanging out with or ignoring another guy, often accompanied by homophobic stuff, and in almost every situation like that it was because they themselves were struggling with their sexuality and wanted to block everything like that out. (not justifying, because of course what he did was horrible, just offering a potential explanation). It seems like if someone is genuinely homophobic they usually flip instantly and confront their gay former friend openly (and negatively). But in cases where someone is deeply closeted, it's usually a sneaky 180 in attitude without any notice or confrontation, along with shit talking behind the back of the former friend. It's usually driven by the fear that people will find out they're gay if/because they('re) hanging out with anyone lgbt.

Now this isn't necessarily the case, but because he fits the pattern so perfectly in terms of behaviour, I wouldn't rule it out.