Whether or not he meant it - and I think a 13 year old certainly can mean it - I don't think this should be treated as 'attention seeking' in the negative sense that phrase usually carries. He sounds like he is very confused, finding things hard, and maybe depressed. You do need to let him know you know, gently, and make sure he knows you are there to support him. Being 13 sucks. Being 13, wondering about your sexuality, and being given a hard time in school about it sucks even more. He needs time to figure it out for himself, to know that whatever he concludes it's ok (and you sound like you'll love and support him no matter what, which is great; he's probably feeling very insecure though so it might not hurt to reiterate it), and probably some adult help to make things better for him.
If there is homophobic bullying going on at school, that needs to stop, and I'd talk to the school. Your DS doesn't need to know that you have, and school can be sensitive about it and make sure that whatever crackdown they have isn't linked to him.
If he needs information, to help him feel less confused or to reassure him that it is ok if he does not like girls in that way, or if he likes both girls and boys, or whatever, there's a lot of good websites out there; you could find a few for him (so he doesn't end up anywhere non-age appropriate, which is a risk with googling) - likewise if he's just scared about what people will think; in that case have a look at It Gets Better.
If he is depressed and anxious in general, he may benefit from some counselling, either via the school or the GP. As I said, being 13 is pretty shit in and of itself sometimes, so he may just be having a bad time, or there may be more going on; you won't know unless you can get him to talk to you.
Good luck. Sounds hard :( but you sound lovely and having lovely parents will be a great help.