I'm going to try to sway my boyfriend's mother from forcing him to cosplay for his 18th birthday
Why don't you want him to cosplay for his birthday?
He's been doing too much cosplay recently.
It is not the parent's role to tell near adult kids how to act or dress. It is not right at all.
I can't let this happen. All his friends are going to be dressing up and my boyfriend was going to look like a loser.
It is ultimately your boyfriend's choice if they want to cosplay. You could cosplay with them to show them that you support their hobby. If it makes them happy, then you should celebrate their birthday together.