Assume nothing, and let him set the pace
With no history, you do not know whether this dog was bathed roughly, sprayed, never washed at all, or is simply bewildered by a new home's bathroom. The safest assumption is that every element is new: the slippery surface, the echoing room, the sound of the tap, water arriving from above, being lathered, being rinsed. A first bath built as a staircase of tiny, stoppable steps protects you both — if he struggles at any step, you have learned something valuable without having soaked and soaped a panicking dog you now must rinse. There is no deadline on this bath unless he has come to you with something harmful on his coat, in which case ask a vet or groomer for help the same day.
Steps one to three: room, sound and surface, all dry
Start away from bath day entirely. Scatter treats in the bathroom doorway and then inside it across a few casual visits, letting him wander in and out freely. Next, with him in the room by choice, run the tap briefly while treats appear, so running water starts to predict food instead of a soaking. Then work on the tub or shower tray itself: a non-slip mat is essential, because one slide can poison the whole project. Invite him to step in dry, reward heavily, and let him step out — repeat across sessions until standing in the dry tub is dull. Some dogs need a ramp or a walk-in shower approach rather than being lifted over a high tub side; lifting a stiff, frightened dog into a tub is the moment many first baths go wrong.
First water touches paws and legs only
When dry standing is calm, add a small amount of warm water to the tub floor or wet his paws and lower legs with a jug, which is quieter and more controllable than a spray head. Test the temperature on the inside of your wrist; it should feel comfortably warm, never hot. Watch his whole body, not just whether he stays: a dog who freezes, pants, licks his lips repeatedly, tries to climb out or goes very still and tense is telling you to stop, dry the paws, pay him generously and try again another day at an earlier step. A dog who shakes, glances around and then settles can usually continue. Keep talking quietly and keep treats coming at intervals through every stage.
The first real wash stays deliberately short
If paw-wetting goes well, in the same session or a later one, wet him gradually from the neck back, keeping water and shampoo away from his eyes and out of his ears. Use a small amount of dog shampoo worked through with your hands rather than vigorous scrubbing, and rinse thoroughly but efficiently — leftover shampoo left in a coat causes itching that he will associate with baths. Skip anything non-essential this time: no nail trim, no ear cleaning marathon, no long blow-dry. Towel him, let him into a warm room to finish drying, and give him something enjoyable to do, because the memory of the bath includes how it ended. A short, slightly incomplete wash that he tolerated is a complete success for a first bath.
Build from the bath he actually had
His second bath should start one step earlier than wherever the first one ended comfortably, not back at zero and not ahead at the difficult part. Baths can stay weeks apart while trust builds; a rescue dog does not need frequent washing, and a wipe-down handles most mud in between. Record the step he reached and what helped in KinsLe through Care Teams, so anyone bathing him next time follows the same staircase instead of guessing.