Seems like I read that Benadryl is totally off limits for cats. While vets approve more human meds for dogs the only ones I've ever heard for cats are a few antibiotics, one specific antacid and two topicals.
You can often get nutrition into a congested cat by making a strong chicken broth (no onions) and giving it to them warm.
The best thing to do would be to get him to the vet, Amoxxi shows results pretty fast.
Jennie, Gary & our kids:
cat colony: Hunter S Tomcat, Archie and Mehitable, Witch, and Calamity Jane, Cuddlebug, Havana, Ghost, Spitfire Liz, Little Bit, Jesse, Moe, Cali, Stardust, Anastasia & Christy, Sally and the 5 boys
dog pack Heinz and Heidi
rip Tiger, Rabbit, Two Stars, Angel,Elaine, Mungo Jerry, Tex and Ijimus. I'll meet y'all at the bridge.