My wife wants her best male friend to move in with us

I'm a man in my early 40s and I'm married with two children. I want to get your advice on my wife's best friend, who happens to be male, and they've known each other since they were 18.

- 

I love the guy - we get on great, he’s a brilliant mate to my Mrs and our kids love him, too. However, he's always around.

He’s the eternal bachelor and every time he’s dumped by (or dumps) a girlfriend, he’s back in our lives almost 24/7.

Recently, he left his job and ended his latest relationship, and my wife offered him the annex in our garden to stay in until he’s sorted.