Yeah totally agree.

You have absolutely nothing to lose by giving him a ring and, as LightOn said, HE initiated it by asking/giving you his number. Call him up and just say something like 'hi there, just found ya number, was wondering how you're doing?' and take it from there. His responces and tone should give you a fair idea if he is indeed back with his ex or another girl... but if it's all good... well take it from there...