How tight are the leggings? If loose fitting like slimline jersey trousers I don’t see an issue.
Lots of designer brands and Scandi brands make boys leggings or unisex. Plain, bright, stretchy cotton or black with a silver stripe and fake buttons, why not?
If they’re more like tights/underwear I think you need to size up or choose a different style. Lots of girls leggings are designed to go under skirts and dresses or tunics. They’d look bizarre with a T-shirt unless your son is a baby.
I love the trend for knitted stretchy leggings on crawling babies as they’re so practical, no runching or bunching or tripping babies up (especially when they practice walking or pulling up). Past the age of 2 not so much.
If your family are objecting have you tried loose fitting cotton jersey leggings in a dark colour? Or slim leg lightweight joggers or skinny summer joggers (I can hardly tell the difference)? My DD has some denim jeggings she loves, they’re basically leggings with thicker fabric and a denim print! She goes through the knees of thin leggings a lot.
Personally I don’t like patterned leggings or joggers on boys OR girls past babyhood or the young toddler stage as they resemble PJ pants. I know they’re marketed as leggings but you still get silly comments like ‘forgot to get dressed did you?’ which seems unfair to the child.
Any leggings that are getting worn or short or a bit tight I put in DD’s PJ drawer if she still likes them.
Also I think you need to style leggings differently to joggers/jeans. They good with ankle boots or wellies or high top trainers. Not so good with sandals or shoes.