I’m not sure if this applies to you or not, but I’ll throw it out there because I see it often.
As a pitching coach the best advice I can give to any dad out there is to be his catcher. Do not try and be his pitching coach. You want your son to look forward to playing catch with you. Just let him throw. Teach him to be natural, don’t think, just let it rip. Teach him that if he makes a bad pitch to forget about it and move on to the next one. Encourage him just like a catcher would do. He is probably very low on confidence right now. The best thing you can work on is that. This may be the first time in sports he has struggled. Too many times I see dads trying to critique their kids after any bad throw. Trying to coach them up. Typically this leads to frustration for both of you that only makes things worse and worse. Does he get down on himself pretty quick when he is pitching? How is his body language? Does he still look forward to pitching?
Being that he is only 9 and 65 lbs there are going to be strength and mechanical deficiencies that are inevitable. Let him use his athleticism, get his body moving fast and throw hard. Use your experience as a coach and athlete to build his confidence and teach him to have a short memory. That may do a lot more than any mechanical advice we could give. Good luck!