there is a 50-50  probability of your son to get selested as per the previous record of the cutoff. the cut off basically fluctuates year by year but there is more probablity of getting selescted as last year the cut off for mechanical in IIT Roorkee was:- Gen -72.47, OBC - 65.27, SC - 48.72, ST - 48.82, PD - 54.14
cut off for 2016 was :- Gen - 65.13 , OBC -58.86, SC - 43.83, ST - 43.83 
cut off for 2015 was :- Gen - 70.88, OBC - 63.84, SC -47.63, ST - 47.78