Hello. Thanks for the question.
Blood coming from the mouth can be due to one of a few things:
1. Dental disease- a tooth root abscess or gingivitis can lead to bleeding from the gums. This would be my main differential given his age. If that's the case, then some antibiotics for a few day and then a dental cleaning with extractions would be ideal.
2. Platelet problem- there is an auto-immune condition where the body destroys it's own platelets. We can see bleeding from the gums and bruising on the gums and skin- they look like red dots on the gums, ears and abdomen. The condition is called IMT. Here is a nice handout on the condition: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=1412
3. Rat Poison- lastly, rat/ mouse poison can lead to bleeding. If there is any rat poison around the property, then it should be ruled out.
For now, I would get an exam to figure out the cause. Hopefully it's just dental disease and they can start some antibiotics to get it controlled until the dental procedure can be done.
I hope this helps, let me know if you have any other questions.