Yes, he will need a visa (B-2 visitor visa) to enter the US, which costs $140.
However, if he has a conviction, he will need to specify this when he calls to make the visa interview appointment and will have to attend one of the appointments reserved specially for people with convictions... unfortunately though, the waiting time for those appointments is usually at least 2 months. In comparison, appointments for normal visitor visas (without convictions) only have a waiting time of about 4 days.
When will they be travelling? If it is in the next few weeks, he may not be able to get the visa in time. I have a drink driving conviction and applied for a visitor visa in 2008. When I called on April 15th to make an appointment, the earliest date they had available was July 1st (the later in the year you call, the longer the wait time, due to the number of people wanting to travel in the summer). Also, when I called to make an appointment for my student visa in 2007 (just after the conviction) I had to wait 2 months as well... I called at the end of September and the earliest appointment was the end of November.
Also, if the conviction was in the last 3 years, he will be required to have a blood test done by one of the 2 embassy-approved physicians in London. The blood test costs between £150 and £200 and it can take up to a month to get an appointment after the visa interview (although I was lucky and got both my blood test appointments on the same days as the interviews) , and then a couple of weeks or so for the visa to be issued (he will need to send his passport in when they request it).
All in all, he would need to start the visa process about 4 months before travel to make sure he got it in time.
