Any suggestions? Ive an almost 16 yr old beagle Smokey DOB 6/29/91 & had him since I was 3 & he was a pup..?

Later he developed at 11ish arthritis & has been takin rimadyl when needed I found out he has a heart murmur - aparentally born w/it. Now he has Lame Legs where you push on a part of his spine his back legs'll drop. He was doing fine on rimadyl &I went away for Lake George on my birthday. I got back & he seemed depressed (yet usually when I get back from trips hes so excited and jumpin all over me). Hes recently havin breathin problems - pantin in a way (which might be showin pain which Ive heard pantin is sometimes a sign of it Its not a "I'm-hot-let-me-cool-off" type of pant. Its when he even sleeps now which I think the heart murmurs kickin in (since hes never had meds for his heart murmur). His back legs are problematic He doesnt sit normally most of the time now. 1 leg sometimes is straight out & the other sittin normal (for ex:). I think it might b time to put him down, but its hard hes been in my life, my shadow & my love- a son. I kno its better to take him out of pain