Hi my ds didn't actually have meltdowns at 5. He was completely passive.
He's very cognitively able but he just didn't really engage with others. Couldn't hold 2 way conversations, cannot read facial expressions, tone or body language. He couldn't describe how he felt beyond happy or sad which is why we worked on expanding his description of how he felt.
It maybe change of routine that's triggering his upset. If it's escalating and going on for this amount of time it's likely meltdown which is harder to manage.
So a weekly visual timetable showing each day and what will happen each day and a reminder each day may help him manage.
Schools often say children are fine which usually means they don't cause an issue. My ds was fine at school in respect you'd probably not even notice he was there as he was so quiet!
I'd ask for a meeting with teacher and ask direct questions you want answered such as "does ds settle as well when the activity changes from routine". "Does ds play with friends or alongside them. Or does he move play towards his interests"
It's very hard to describe sometimes what particular things make you think they are autistic but me and my family just knew ds was from about 18 months. Although no learning difficulties he just was quite typical.
If you have concerns approach the GP and ask for a referral to a developmental paediatrician. I always think never underestimate mother's intuition!