Realistically speaking (ignoring the yaoi biology), is that dangerous?? Like he can actual...

It’s like he gave him some dangerous enema

Enemas are common a lot of people do it but extreme enema inflations like these needs a bit training so don't do it.

short answer, yes

So in our sweet airhead’s case where he doesn’t have any training, might potentially be fatal?

maybe all that sex counted as training

I wouldn't say fatal in a way of straight death but there would be health complications which could be fatal if it develops further such as colon damage or infections.