Something similar like this happened to my daughter at that age and I found out she had been bullied for years by the same group of girls in her school.
You really need to talk to him and see what's going on. 9th grade is hard because they're all of a sudden in high school and their grades count towards college and their teachers are expecting more out of them. He may feel stressed and/or overwhelmed. Please try to talk to him in a non confrontational way and see if he will open up to you. I wish you the best. It's not an easy age.
any changes in friends?
Sometimes boys get mixed in with a bad bunch of teens and sometimes this group is not the athletic type- maybe into a bit of drugs and alcohol?/???? Caring for their bodies and the idea of self-discipline that athletics calls for is not what they are "INTO"- they would prefer to be irreverent and easy going- self centered most of the time.....
SO there are several things that may happen next- and one of them will be sliding deeper until he hits rock bottom- No mom wants to watch that- sometimes it's a wake up call but as I said- we don't want to wait that long---- he is still relying on you to guide him
dad could be extra hard on him and insist he behave/ is that an option? Sometimes these boys are craving boundaries and go back and tell their loser friends- " ummm my dad said he'll break both my legs if I hang with you any more" of course dad doesn't mean it- but the boy realizes his parents have had ENOUGH!!!!
Watching his every move and spending more time with men that share high morals and self discipline would be an excellent start for the boy- Martial arts?
Work his buns off at home- keep him busy- let him earn back his freedom a little at a time- read on the net about tough love-
You are wise to get ahead of all this now- hope he is not already bigger than you-
I've seen this happen to boys who have been through trauma - what's your best guess as to why he suddenly has changed?
I've lived in the same house all my kid's lives, and have seen boys suddenly take a terrible turn and stop caring. I've also seen boys mess up and get scared enough to get back on track.
Do you know from him what happened that caused the change?