It will not necessarily bar him from entry. Entry to the US for people with convictions is, I understand, discretionary.
Perhaps - apparently he would no longer be entitled to enter under the 'Visa Waiver Program'. And with a name like Hussain I don't fancy his chances of getting a visa (especially with a conviction for a violent offence).
http://www.fco.gov.uk/en/travel-and-living-abroad/travel-advice-by-country/north-central-america/united-states?ta=entryRequirements&pgSeveral million British nationals travel to the US annually under the VWP without any problems. Only people described as a "British Citizen" on the photo page in their passport qualify to enter the US under the VWP. If you are described as a "British Subject", "British National (Overseas)", "British Overseas Territories Citizen", "British Dependent Territories Citizen", "British Protected Person" or "British Overseas Citizen", you will need a visa.
You will also probably need a visa if you fall into one of the following categories (Note: this list is not exhaustive).You intend to stay longer than 90 days
You intend to travel to the US for a reason other than business, pleasure or transit
You are conducting official government business
You have been arrested (even if not convicted)
You have a criminal recordYou have a serious, communicable disease
You have been refused entry to the US on a previous occasion
You have been deported from the US
You have overstayed during a previous visit