If you are really interested in him why do you feel the need to make an excuse? Are you sure there isn't something about him that doesn't make you uneasy? I knew a guy once that loved to party, he was cute and always asked mw to hang out. Something always came up and so I couldn't. I always had an excuse. I find out after I moved that this guy killed his girlfriend and stole her car and money. So I wasn't aware of my reasoning on a conscience level but luckily my intuition picked up on the fact that something was off with this individual and I was safe for it. Re-examine your motivations for making excuses. If they are truly becasue you are shy or embarresed just say so to him - I am shy around you because I like you- if not you may be unconsciencly avoiding this person for reasons you don't know yet.
Say the excuse was a joke and proceed with the plan.