Sorry no one has responded to your post until now so hoping i can help. Welcome to the asd forum.
Sometimes cases of autism can be easily spotted other times cases can be missed eg if people can mask so appear to function in the world (in some cases socially). No friends in school and being a loner (sometimes by choice) can be a sign of autism. By issues with routines are there some things he has to do a particular way over and over again? If you tried to teach him a new routine that he didn't know (or change a routine he knows very well) would this upset him?
I can mask (to an extent) so can appear to understand others feelings and pick up on social cues but it comes at a cost, I need quite a bit of alone time in order to process what's going on. Being hyper aware of others is another sign of autism. He might be concentrating so intensely on whats going on that starting a conversation is difficult for him. Knowing how and when to start a conversation can be difficult for autistics. He might be so self conscious of not interrupting that starting a conversation with kids his own age becomes too much for him. Conversations involve a lot of quick decisions on what to say, when to say it, how to say it, what not to say, how to start talking, how to stop talking, how long to talk before stopping to give others a chance to speak. He might find people too confusing to speak to or it could be anxiety that's preventing him from talking. https://www.nhs.uk/conditions/selective-mutism/ but you can have selective mutism and autism.
Autistic children can struggle to make and maintain friendships. Are there any kids in his class who have similar hobbies and interests that his teacher could buddy up with him or introduce him to them? Maybe there's kids in his class who are poor at something he's good at? Could he perhaps help the other kids with their drawing? (you mentioned he was creative or the other creative activities he can do) Then they could ask him specific questions about drawing to get him talking about something? i liked maths so brought some maths books to school and shared them with other kids. i also shared apples from grans garden but was unaware that some may have maggots in them at the time of sharing causing embarrassment when the kids found out. i was 10 at the time and had no diagnosis at school. i also used to just walk around the yard at school playtimes.
i was severely depressed from age 8 but was unable to express this in ways others noticed. It was the 1980s and childhood depression was less well known about then. i still have bouts of depression but at least i know why and have some strategies that can help.
The term 'high functioning autism' is quite misleading, i could go to school, write in my book, read, but playtimes i either walked around the yard or put a coat over my head sat on the floor in the playground. i lasted as long as i could in the dinner hall, (unless the dinner lady banged a spoon on the table telling us it was too loud) because i didn't want to go out and play, dinner times were predictable, playtimes were busy, sensory stimulating in a bad way, overloading and too much. There are different presentations of autism, some kids are too chatty, even with strangers like me, others are more like your son, then there's kids out there like neither of us.
i didn't know what i had at the time of school, due to delays in support and diagnosis, i have only become more settled in myself in my early 40s, my diagnosis was in my late teens. There is a lot of overlap between my diagnoses and disabilities, i have fibromyalgia a physical health problem, but that causes concentration issues but so does autism. i have elhers danlos another physical health problem, but that caused my physical developmental delays as did my autism, i have dyslexia so my reading is slower the right way up than with inverted text (very annoying when quiz answers are written inverted). Secondary anxiety and depression can accompany 65% of autistic kids (NAS, You need to know campaign).