Dear parent,
Thank you for your query at DoctorSpring.com
I can understand your concerns. It must be really disturbing to find blood stained saliva.
The most common cause is due to the child biting tongue or lips during sleep. But, the fact that there are no injury marks on his lips or tongue obviously rules out that cause. Secondly, the source can be internal stomach bleeding. But, before thinking about internal bleeding, we obviously need to rule out basic mimics. Chocolates or other foods such as brownies or cakes can stay back in the child's mouth and present like this.
If it is indeed internal bleeding, child would need to be examined in detail by a paediatrician. We need to check for any bluish bleeding spots under his skin. We need to check for enlargement of his liver and spleen. We also need to get a few blood tests such as platelet count and coagulation profile done.
Hoping it is not actually internal bleeding.

Regards
Dr. Saptharishi