He's got a pretty good vocab I would say, it's more the articulation and pronunciation of words.

I've been trying to figure out if it is new words or words he used to be able to say clearly. I think is it more than just new words.

It used to be that family/friends would understand him 95% of the time, and now it is getting to the point where it's only me who can understand him that much and others struggle with what he is saying, probably only understanding 75%. He uses sentences etc, it's just that he is not making the sounds clearly. It is definitely a regression, rather than a developmental delay.

He hasn't had his hearing checked.

I am getting a little worried out that he may have some rare brain tumour or something (end freaked out mother rant).