I need advice.

I'm a full-time student employee working as a Software Engineer. By 'student employee' I just mean I get paid less. My boss wants to keep me long term. However he is very nice and has said he would not be upset if I left any time. I plan to leave in August, and I don't know when to tell him.

He keeps nagging me about applying for the full-time position, which will be exactly the same work but almost double the pay. However I am hesitant to do so because I know I'm leaving. However I can't tell him why I haven't applied yet, because I don't know when to tell him I'm leaving. I've read on here that you should wait until the last minute to apply. So as I see it I have a few options...

• Tell my boss I'm leaving and ask if I should still apply (he might say yes, who knows?)
• Don't tell my boss why I haven't applied, never apply, and then just leave in August. This would not be great because he would continue to nag me about it and wonder why I'm not doing it.
• Wait to tell my boss until a slightly later date. This wouldn't really change the situation, just put it off further. I'd still have to make the decision.

I have no idea what to do. My boss seems nice and friendly, but they haven't payed for my school yet and I'm afraid he will change into a completely different person and then refuse to pay or make my life difficult. What to do? This situation is really unpleasant. I would really appreciate any input.