This is Dr. Christian.
Thank you for the information. Here are some possible causes:
In this case I think you can wait until morning. The metronidazole was a good idea unless he has lost or gained a lot of weight since that prescription was given. More than likely he has colitis but it can still progress to HGE. If he becomes more lethargic, starts vomiting again or the blood increases then I would have him checked tonight. If you have more questions please let me know.