Sounds about right to me, esp. if he feels well and has no problems that could be related to his heart. 17 is not quite an adult yet but I've known really good distance runners with RHR in that zone. I only got down to the mid 50s myself, when in my 30s and running .... sigh.
What do the rheumy/pediatrician say?
Jen, 42, happy partner of James and Moma to Evan, 14, & Lucy, 12.5 (Crohn's dx @ age 3; on Remicade since April 2010.) I take piroxicam, Flexeril, & Nucynta ER nightly. 3 anti-TNFs didn't pan out for me.
"Science is the father of knowledge, but opinion breeds ignorance." -- Hippocrates