limit on amounts of visits to USA in a specific time period

My boyfriend came from the UK to visit me in the US in January and stayed for 3 weeks, he now plans to come back in a little over a weeks time to stay for a further 2 weeks and I was told that you're only allowed one entry into the USA every 6 months

is this true?

kind regards

-Heather

On the visa waiver he can come in to the USA for 90 days at a time then he can come in again for another 90 days this is though out the year 12 month period as long as he only stays for 90 days or less each visit on the visa waiver he is ok.