100% This ^^
You have a contract with them to work however many hours per week in return for your pay. You owe them nothing more than this. Even if your boss is a mate, he, as a boss, should be pleased if you're bettering yourself elsewhere. That means he has done his job as a boss - get the most out of you (from the sounds of it, he's achieved on that front) and then let you go on to something better.
If you leaving drops him in it, it means he hasn't been doing his job properly.
Fix yourself up with something concrete, then tell him soon-as. That way, you can play the old "I respect you, so I wanted you to be the first to know" card. With a bit of luck, he'll have already handed over the cup final tickets by then.