I agree with Emmeline that it may be a bad fit. Has he said why he doesn't like it? Sometimes kids are upset because they think they're missing something, like time outside, or a TV show etc. Sometimes the therapist ahs a personality that can be difficult, or the child can feel frustrated by having to work on something that is so hard for him. Have you talked to the SLP about how he feels about therapy? Does he show these feelings during sessions, or just prior to/following? Some kids don't want to go, but once they are there do OK. Is she willing to change anything to accommodate his needs?
I wouldn't be shocked at no carryover yet - articulation improves in stages, and first words and short phrases improve, then sentences and conversations. Has she given you homework to do with him? She should be providing you with a home program to help practice and generalize skills. She should also have identified goals for him, for the insurance company, stating what he will do, and in what time frame. You can ask how he is progressing on those specific goals, to get more information. Something like he went from never saying "s" to saying it 10% of the time but not in full words yet, only on its own or something like that.
If you need to take a break for the summer, don't feel bad about it. Kids can need breaks from therapy just like they need breaks from everything else sometimes!