My friend started a band and got the people in it excited about it, then starts talking about joining another.

So a couple months back, a couple friends of ours got a band together and were doing real well. So my guitar-playing friend starts a band of his own with some other friends of ours and we had lots of good practices. However, summer came along and it's become hard to put some meetings together. Then, today, that other band I mentioned earlier recruited him to be their singer, and he's talking about taking it. I was completely blown away by this. How can I convince him that he needs to honor his commitments?