Overbearing Parents how do I tell without disrespecting them

I would be interested why your parents don't like his parents?
How about his parents? Is their feelings mutual or do they have any comments to make about your parents dislike of them?

These sort of enquiries need a measure of diplomacy but mostly I feel plain speaking is best, avoids misunderstandings and is difficult for anyone to ignore without a display of very bad manners...

If your parents have a good reason, not only for disliking his parents, but for "visiting the sins of the parents on the son", you need to consider the possible consequences of continuing your relationship with him...

How important is your family to you? If they ostracise you will you lose any sleep over it?
Could you move into your own accomodation any time soon or perhaps live with his parents for a while, or friends, or other relatives?

If there is not a tangible reason why there is bad feeling then I suggest you continue with your relationship and don't worry too much about disrespecting them - the disrespect will then be mutual as you have reached your majority and your parents cannot and should not attempt to control your choices outside the home...
Respect cannot be demanded - it must be earned - AND it is a two-way street!

Until you gather the information the bad feeling could be caused by some silly rivalry or past slight or have more serious causes, such as an extra-marital affair between your father and his mother or vice-versa meaning you and your boyfriend might be siblings!

I strongly suggest you keep an open mind until you have all the information. Their silly attitude might be misplaced or misguided protectiveness or it might be stupid prejudice...

No question about it though, you have a right to put yourself first in this matter...