Why is my dog throwing up in the mornings?

I have a boxer/mix. I really cant say how old he is seeing how I got him from the pound. I have had him for going on 3 years now. The vet says he is no more than 4-5 years old... I have noticed on occasion spots of yellow residue in his cage when I get home from work. I am uming he is throwing up and then eating it... This weekend, in the mornings he has been asking to go outside just to throw up. Its just yellow bile, not food... He still poops and pees as normal... Any suggestions?