Shih tzu infection - what is it?

I have a 7 year old Shih tzu, Bailey...for the past day he has been realy lethargic, doesn't want to move, doesn't get up when i come through the door, usuallly he's full of pep and extremely happy to see my husband or I get home. The same symptoms occurred about 5 weeks ago. I took him to the vet and they found nothing. What they did find was he backed up with poop, so they sent me home with lactalose. They also pumped him with fluids in his back and sent me home with clavamox. They couldn't identify the infection but said he had one...?? Anyhow, the same symptoms are back so i've been giving him clavamox twice daily and also the rest of the lactalose. He seems to get better throughout the day and then goes back into a funk. I just noticed yesterday and today some thick goop on the tip of his penis. I've cleaned it off both times but I'm not sure if perhaps he has a penile infection? It does have a strong condensed uring odor. He's eating his food twice a day, drinking water and going to the bathroom regularly. Any insight would be greatly appreciated. Regards, JI