Hi and WELCOME

. You can easily find out what's going on by putting some stool in a plastic baggie and bringing it to the vet for a fecal test - which you should do asap. He's probably in rough shape if he's had diarrhea for 2 weeks. It could be giardia - and at his age (bordering puppyhood, but an adult really) - he may not be able to fight it on his own like most adults, and may need meds. It could be other things too though, but you really need to do a fecal. Honestly, if it were me and it'd been this long - I'd bring him in.
How is his appetite, drinking, energy, color of gums?