D.W.
Speech Pathologist, Speech Therapist-same thing. Their exact title is Speech-Language Pathologist. They also are called speech therapist. They have a masters or doctorate in speech pathology. A few states employe SLP assistants. But they can not evaluate and they can not work without the direct supervision of an SLP.
You are getting worked with a lot because (presumably) you/dad are the ones with him the most. You have to be knowledgeable about how you can help him develop his speech and language skills within whatever limitations/concerns he has at this time. It is essential that you understand what is going on with him and how you can best address this especially given his age. I think he's around 2.5 right? Children that age have extremely short attention spans for therapy and can't be drilled for long periods. If you are drained and overwhelmed imagine what he would be feeling if they tried to do that with him in a session. It would never work and he'd never make any progress. But if you know how to help him (since you are with him all the time) you can incorporate the strategies they are teaching you into your everyday routines.
He is probably being evaluated often because of the program he is in. It's probably a situation where if they aren't making progress or setting new goals often he will be out of the program.
If you have concerns, then you really need to address those with your son's therapist. He/she will be able to best answer your questions about your son's plan of care and therapy goals.