Rehoming a dog who did not work out is usually discussed in terms of the dog who leaves and the people who made the hard call. Even where the two dogs never became friends — especially, sometimes, where life had become tense — your remaining dog has lost a presence that structured his days: another body at the door, another bowl going down, smells on walks and bedding, and a routine built around managing or enjoying a second dog. Settling him is mostly about steadiness while the household finds its single-dog shape again.
Keep his anchors exactly where they were
The strongest help you can offer is sameness in everything that is his. Feed him in his usual place at his usual times, walk his usual routes at his usual hours, keep his bed where it has always been, and preserve the small one-to-one rituals — the morning greeting, the evening game, the way the night ends — precisely as they ran before the second dog arrived, if you can remember them, or as they ran during the two-dog period with the other dog's parts simply removed. What unsettles a dog after a household change is rarely the absence itself; it is absence plus everything else moving at once. Resist redecorating his life in the first fortnight: no new walking route launched the same week, no house move of his sleeping spot because the room looks emptier, no sudden flood of visitors invited to cheer everyone up.
Let him process the departure in his own way
Allow him to sniff the rooms, the garden, and the places the other dog slept, without hurrying him past or staging emotional tours. If he seeks out the departed dog's bedding, leave it down for a while rather than stripping every trace on day one in the name of moving on; scent fades gradually in a lived-in house, and gradual is easier for a dog than a house that changes smell overnight. If instead he avoids the other dog's spots or seems brighter and looser within days — eating well, playing, sleeping deeply — believe that too, and do not go looking for grief he is not showing.
Mind what your own state is teaching him
Dogs take emotional instruction from their people, and a household processing guilt and sadness moves and speaks differently: quieter rooms, longer sighs, conversations that stop when he enters. He does not need a performance of cheerfulness, but he does benefit from the household returning to its ordinary sounds and rhythms — music on, normal chatter, laughter when it comes — rather than the home staying hushed around the decision. Be honest with yourself, too, about consolation behaviour: constant fussing, hand-feeding extras, and abandoning rules because you feel sorry for him can leave a dog more anxious, not less, because the humans have become unpredictable.
Do not rush to fill the space
However the departure went, give the household a real interval as a one-dog home before considering another dog, both for your dog's sake and for the next dog's. Your remaining dog needs to show you his settled single-dog baseline — how much exercise, company, and stimulation he actually wants — because that baseline is what any future companion would need to fit. If your dog's searching, appetite loss, or low mood deepens or persists beyond a few weeks rather than easing, talk to your vet, since prolonged change after a household loss deserves a check for pain or illness as well as support for his mood. For relatives beyond the household, Family Sharing on KinsLe gives relatives you select access to his profile, so the family who knew the two-dog home still has a way to see the one who stayed.