If he would consent to let you give him a cleansing enema and examine the return before flushing that would be a good first step. That way you could get an indication of how badly his colon may be blocked with waste matter.
You might want to start with a 3-4 pint mild soap solution, and follow afterwards with a full bag Sea Salt solution to rinse any remaining soap residue.
A lot of times the guys will protest in advance at the idea of an enema, but afterwards, once they have had the experience and feel better, will often request them in the future.
If you haven't ever given an enema to someone previously the following links should help: