10 month old randomly hates the car seat?

could you put him in the front so he can see you? you can do this in most cars as long as you can turn airbag off. it’s not any less safe.
Also worth getting a fit check? Maybe he’s needs headrest adjusting. There’s a lovely group on facebook called bump to booster uk car seat seat support

Is he feeling unwell, have a cold etc?

My boy did exactly this for 2 days no apparent reason then he started getting cold symptoms and was so clingy, cried every time I put him down and ended up vomiting on a few occasions due to becoming so worked up.
He was like this for just over a week until he then felt better and the cold cleared.

At his development check they said its pretty normal for babies to go through a stage of things like this at this age. My boy is also 10 months old

Babies are people too sometimes I am okay with something one day but the next heck no 🤷‍♀️ maybe ?

Mine did that for a few months and then grew out of it thankfully, but I don't know what was causing it, we didn't go in the car much luckily

This is pretty normal. Most young children go through an "I hate the car seat" phase at some point. Mine did it from about 9-11 months.

Maybe he is adjusting to his new car seat…we went from the infant car seat to the car seat that grows with them. When we put on one of her go to songs and she has a robot snack she is okay most of the time