@rotc128
1. He can enroll in any ROTC program and pursue a commission without a scholarship. This is the path my DS took as an NROTC MO and he was awarded a scholarship in the middle of his sophomore year. Some classmates made it all the way to commissioning without a scholarship.
2. All services have some form of OCS/OTS, or whatever that service calls it, to commission folks who have already graduated from college
3. ASVAB is not required for officer commissioning programs. They will use the SAT/ACT scores. DS got a 92 on the ASVAB while in high school, but it was never taken into consideration so far as we know. I think the ASVAB might be used as a substitute for a SAT/ACT score in some cases, but not sure about that. I also would not know what the minimum score is, but I'm confident a score of 31 would not be competitive for an officer program.
4. I know folks can commission into the Army Reserves if that is what he wants. Folks commissioning into the Navy and Air Force through their ROTC programs go active duty. I do know a prior enlisted Marine who commissioned through OCS, after completing his college degree, who commissioned into the Reserves. I don't know if that was because he was prior enlisted and had already done his active duty stint, because of the way he commissioned, or because of the odd personal circumstance he found himself in at commissioning. If your son goes this route he should confirm his opportunity for the reserves with the OSO or ROO or whoever he speaks with if he is interested in that.