I would think lack of response to his calls would be enough to get his mind going to thinking you know what's going on and he may very well 'come clean' on his own.
Another thing I've seen and heard is to not let him know what you know, just that you know.
I would certainly not insert your thoughts on what he needs to do about it, ie therapist or anything else, that will only make matters worse, all you can really say is 'you need to figure out what it is you need to do' and walk away. Given it's a LDR, that may work in your favor as you don't have day to day exposure to one another. Not sure.
Me 35/XW 33 S13 & S12 M: 10/17/98 OM & S: 07/08 D final 06/09/09 12/03/09 - 06/13/10 "Piercing" 06/13/10: Engaged to Re-marry 10/17/10 06/25/10: Expecting baby #3 2/14/11