Boxer losing hair

[smwsmw]

I have a five year old boxer who is the love of my life. On his back and sides towards the rear end of his body, his hair is thinning a bit. I was thinking that it is from him laying on the couch, the bed, the carpet, the chaise, etc., but I haven't noticed this before in any dogs I have ever had, and I've had a few. He was throwing up the other day, and I just thought today that they could be related somehow? I will call my vet tomorrow, but was wondering if anyone had any opinions. He is not scratching abnormally. Thanks!