I think its a great idea. I don't know why you would actually think his ex would buy him a father's day present lol. My ex and I have been seperated for six years and she's never once bought me a father's day gift, or vice versa. I wouldn't expect your bf's ex will get him anything either.
Like I said, your idea is splendid, but if you want to give him a present that's guraunteed to please, bring home your best-girlfriend for a night. As a guy, I can truly say that would truly make my day.