Can’t put on any weight, 14 years old, lab mix

Rowdy is approximately 14 years old; has been with me 13. About 2 years ago, he began losing weight. He’s short-haired, probably Labrador mix. He was weighing 60-65 lbs. but is probably closer to 55 now. He’s been wormed annually in June. He gets immunization shots (5-1) and rabies shots. He lives on 1-acre with me, 2 horses, 3 chihuahuas, a pit bull, a shepherd mix and a Catahoula Cur, none of whom are off the property except for rabies shots or injuries. Although he has maintained his appetite, there’s no “meat” on his bones. He has lost strength in his hindquarters, although he is still able to rise, lope off and “hold his own” among the other dogs. He gets 1 or 2 cans of Purina Pro Plan dog food each morning and I’m experimenting with high-calorie supplements. His urination and bowel movements are regular and he does not drink an extraordinary amount of water (admittedly, he is not observed every minute of the day and has free access to both dry kibble and water). Other than a veterinarian’s exam, can I do anything else to promote healthy weight gain?