I must first say, I have no expertise at all in legal matters, so what I'll say here is by no means to be regarded as "gospel."
But, it's my layman's understanding that if he committed no further crimes beyond 21, and he is now 35, this record may actually be considered "spent" -- I forget the official word for it, but it's when your slate is considered wiped clean because the crimes were so long ago, and also they did not involve violence, manslaughter or murder, which obviously probably do not ever get cleared from your name.
Obviously if a crime is recent there is no question that he will need to apply for a visa.
But when a certain number of years have passed on crimes of a non-violent nature, the situation MAY be different and he may now be considered "clean" -- you will have to get pro advice, and it's worth it to find out, as you MIGHT not have to go through the visa thing after all. I must emphasize I only say MIGHT.....
I will tell you something ---- my now-ex boyfriend had the same record more or less as your man. And all before he was 21. This is a UK person.
We were worried about this too, when he was about to visit me in the US. He asked a couple of friends of his who had also traveled to the US and whom he knew also had juvenile "form". They said they had no problem entering on the waiver as they found their own records were considered null and void by the time they were near middle aged men with no further crimes.
So my boyfriend came in on the visa waiver, twice. No problems whatsoever.
PLEASE do not take this as gospel -- I would ask a professional about this. MAKE SURE the "spent" issue applies to your man's situation.
But just as a non-expert with a boyfriend who had a similar situation and was fine, I just wanted to put my mention in of the fact that some records expire after a certain amount of time especially if the crimes were considered committed while not of an adult age.
Again, get advice on this, but it's possible his record should not even be on the "books" anymore; my boyfriend's wasn't any longer, at the age of 46.