Is it bile (yellowish liquid) that he/she is throwing up? If so, it's probably just because he is hungry and has an empty stomach. Try varying the times of day that you feed him so he doesn't get used to eating at an exact time every day. Also, giving a treat or small meal before bed usually helps eliminate this problem as well..
HOWEVER, I do agree with dege and TWAB that the first thing you should do is go to the vet. It may be something completely different (and dangerous) other than just bile and it's always better to be safe then sorry.