13 yo had heart murmur 2 yrs ago otherwise healthy& active today had dental cleaning w bleeding should he take profilact abx? what to look out for?

Probably not: Only if there is known structural disease of the heart is antibiotic prophylaxis indicated. You'd physician should have advised you.

Usually not: Pre-dental antibiotics were once widely prescribed but the list of indications has narrowed considerably. The list today includes those with artificial grafts/valves, complex congenital heart disease, etc. Your simple murmur does not qualify.

Heart murmur: If the heart murmur was determined to be significant and indicative of valvular abnormality, then prophylactic antibiotics for dental procedures are important. If the murmur was proven by echocardiogram to be only functional and not indicative of structural valvular disease, antibiotics are not needed. Such murmurs often clear up.An evaluation by a cardiologist may be best to answer your question.