Probably useless, since i'm in the UK, but i have a friend who missed all HS from age 12 onwards due to illness and then did a 1-year "access to arts" course to gain entry to a (very good) university. But she did this as an adult of course. The universities here will only take students under 16 with a chaperone and in very unusual circumstances (i used to work in accommodation, which once had to house a 15 year old russian genius who had only been admitted to begin physics because his birthday fell within the first semester).
Does your son know where he'd like to go to college? Perhaps you could contact specific schools about this. There are almost always ways to get around things. I also have a friend who was "homeschooled" for high school (she says her mother's idea of homeschooling was to give her the bus fare to the library every day, so not homeschooling in the sense probably many many people here are doing it) and she too just picked a university, enquired as to what she'd need for entry (for her they wanted exam grades since she was at an age of entry where others were coming from HS) and went off and arranged an opportunity for herself to sit those exams. She's a PhD now.
Does your son know where he'd like to go to college? Perhaps you could contact specific schools about this. There are almost always ways to get around things. I also have a friend who was "homeschooled" for high school (she says her mother's idea of homeschooling was to give her the bus fare to the library every day, so not homeschooling in the sense probably many many people here are doing it) and she too just picked a university, enquired as to what she'd need for entry (for her they wanted exam grades since she was at an age of entry where others were coming from HS) and went off and arranged an opportunity for herself to sit those exams. She's a PhD now.