Why think up any reasons to decline? Everything you've written here regarding your time and how precious it is to you, how you like to have space and time between get togethers should be sufficient as a reason. Leave out how you think that he is trying to hijack your summer. That won't help things nor is it productive but everything about how you feel you need space, don't like to meet people regularly and such should be enough. Be honest, it's always the best way. If he takes it wrongly or is offended by this answer it says tons about how important your feelings are to him and should kind of tell you whether you want to even continue the friendship. If he's a reasonable person and truly a good friend, he'll understand.

Don't make up excuses. eventually you'll run out of them and the truth will inevitably have to be revealed.