ok, things do change when you becomes involed with a new love. and all time and energy is spent with that person.
friends should always be there no matter what , even if your not called up on aas and when you think you should be!
he obviously has invited you, so ofcourse go to the wedding.. and he he has included for you to take a guest, well by all means do so, surely it doesnt have to be a man.. couldnt you take a female friend... he is getting married after all, so im not sure if he would be that bothered on his wedding say if you turn up with a bloke or a woman to be honest.
you have to remember also that weddings are not cheap, and he has chosen a selected few to the wedding... so as you say you were very close, and he has chosen you as his closest friend to share his special day.
go and enjoy, your friends special day, its a day out thats been paid for... and he obviously would like you to be part of that.