Hi.
Summer can be liberating and fun because you often have more time to do things that you might not have the chance to do during the rest of the year.
Your boyfriend sounds like he's being honest and speaking from experience when he says his summer relationships have never worked out. Maybe this is his way of suggesting a breakup. But maybe he's not sure what he wants.
Does it seem as if he would want to stay together, but might be more focused on tennis? If so, maybe you and he could work out a plan where you call him one week and then he calls you, or something like that. Or perhaps after talking, you'll both decide to take the summer off as a couple, but get back together once school starts up again. You should talk to him and figure out where things stand before he goes away.
Consider what's important to you about your boyfriend, your relationship and how you want to spend your summer. It all comes down to what you value.