Yes, if his birth mother was using drugs and sniffing glue during her pregnancy your son is apt to have speech as well as other developmental difficulties. You need a thorough professional evaluation. A speech therapist can provide speech therapy but is not likely able to diagnose the cause of his speech difficulty. I suggest that you find a speech pathologist for an evaluation. Your son's pediatrician should be able to refer you to a professional who could do a more global evaluation as well.
If you know what drugs she was using there are some specific disabilities related to specific drugs. Fetal alcohol syndrome is one such disability.
And, of course, his speech difficulty could be unrelated to her drug use. Many children have speech difficulties, some of which resolve themselves before they start school. They just develop speech later than the average child. But......if he has an anatomic or brain function difficulty it's important to find that our early and begin treatment. My grandson started therapy after an evaluation at 2 1/2. At 6 he's still having difficulty with speech. If he'd started with speech therapy earlier he may have been further along by this age. We do the best we can with what we know. I'm very glad that you've asked this question and encourage you to get an evaluation soon.
As to whether or not you can teach him before his appointment the answer is no unless your appointment is months away. It is important to accept that he is doing the best he can do with his speech and not insist that he do better. If asking him to repeat words frustrates him don't ask. Show him as much approval as possible. Learning and teaching him sign language will be helpful.