Hi. My 14 Yo Male Yellow Lab Isn't Acting Right.

Hi. My 14 yo Male yellow lab isn't acting right. He has has trouble walking for some time, but it has worsened drastically over the past week. He holds his tail down between his back legs and sits on it most of the time. He barely eats his hard food, which he used to love. I made him some homemade dog food this morning and he did eat a cup of that. My biggest concern is that his abdomen is swollen and hard. It appears to be sensitive but he will allow me to gently touch it. What could be wrong?