How do I make him look younger?

No, don't do this. Don't assume your parents will hate him because he's a bit older. Don't try to make him seem different to how he is - he won't appreciate it.

The fact of the matter is you and he are both legally adults and age doesn't matter. If you were 20 and he was 80, it still wouldn't matter (highly unlikely, but the same principle applies).

I recommend telling them the truth before they find out themselves that you have been lying about his age. Age is just a number and if you love the person it shouldn't matter and your parents should understand. But to answer your question shave his face, make him wear younger clothes, and give him a nice haircut and you should be fine!

Don't do that, that's a bad move. You really shouldn't lie to your parents about your boyfriend, because they will get shocked that you had lied to them. Tell them the truth, and suit him up appropriately and tell him of the things he has to be careful for when he's around your parent. Eg, 'My parents hate it when you put your elbows on the table'.