hey,
From what ive read, i think you should go for it. We've all gathered that your not too sure quite what you want from him, and it seems that he's not quite sure what he wants from you. Can you really blame him for being a bit hesitant, he asks you out and you make up an excuse...so hes probably sitting there going, hmm, is she making up an excuse or is she genuinly going to the dentist/hairdressers/mums birthday...or what ever your excuse was. He's obviously giving you the benifit of the doubt, he shows this by repeatedly asking you out even though he may be facing rejection. As for the flirting with another girl, havnt you ever heard of the expression treat them mean keep them keen. He'll have been told (by the media/friends/past experiences) that if you pretend to fancy someone else the girl your actually after will get jelous and want you more. He's probably not even thought that its hurting you/confusing you.
darling, from where im sitting your both wanting the same thing.
Go for it, and if it doesnt work out...its not as though you've lost anything. and if it works out...you'll have a fun summer infront of you!
good luck
love Katy***