Personally I'd get him all in.
IMO these hands come along seldom enough that you have to make them work for you. I like your bet out, often this will be taken as a steal and will get you some action.

The 2 diamonds on board would make me not want to keep it cheap in any way. If he slow played a big pair with a diamond in it he will call (more than likely even if not holding a diamond in a big pair) or he could well be on the draw or as you say could hold A9. Either way I want him in, you have him well covered so it's not a major disaster if he did hold the other J and bigger kicker. He's pretty much committed after his bet so if he didn't put you on a Jack he's in trouble.

It's always possible that he has pocket 9s - if so that's life.