Undo the strangeness first
An unsettled rabbit coming home is usually a rabbit who still smells of somewhere else. Boarding surrounded him with the scent of unfamiliar animals, unfamiliar hands and unfamiliar bedding, and rabbits navigate the world by smell far more than we do. So the first job is restoration: his own pen arranged the way it always is, his own litter tray in its usual corner, his hidey house where it belongs, and ideally bedding or a fleece that was kept at home and still smells of him rather than of the boarders. Put him down gently in the middle of it and step back. Do not carry him around the house reintroducing him to rooms; the pen is the message.
Food and water within reach, pressure at zero
Place fresh hay right beside wherever he has chosen to sit, plus his water in the bottle or bowl style he prefers, and a small pile of the leafy greens he likes best. Eating is both the thing you are monitoring and the thing most likely to restart his confidence — a rabbit who eats is a rabbit deciding the world is acceptable again. If he ignores food for the first hour or two in a hunched, watchful posture, do not keep offering, stroking or coaxing; that attention is pressure to a prey animal. Leave the room quiet, let him listen to ordinary household sounds returning, and check back at intervals rather than hovering.
Protect the quiet deliberately
Brief the household as if it were an instruction that matters, because it is: no picking him up today, no visitors leaning into the pen, no children presenting treats through the bars every ten minutes, and dogs kept out of the room entirely. Keep his feeding times at their usual hours so the rhythm of the day matches his memory of home. If he lives with a bonded partner who stayed home, let the partner's calm behaviour do the reassuring — but watch the reunion, because he will smell wrong to her too, and a scuffle is possible; a shared pile of hay in the pen usually lets them renegotiate over food. If he boarded together with his partner, simply let them resettle as a pair.
What to watch through the day
Check two outputs on a loose schedule: is he eating, and is he producing droppings? Both should restart within the first several hours home, and droppings should build toward his normal quantity and size by evening. Also notice posture changes — a rabbit progressing from hunched hiding to washing himself, stretching out or flopping on his side is a rabbit standing down. Refusing all food, grinding his teeth while hunched, a silent litter tray, or a rabbit who feels cold add up to a different situation than nervousness, and they should not be given another day to resolve themselves.
The next-morning line
Set the decision point in advance: if he is still not eating properly, or droppings have not returned to something like normal, by the morning after his return, phone the vet that day and describe the boarding stay and what you have seen. A rabbit's gut punishes hesitation, and a vet would far rather hear about a nervous rabbit early than a stopped one late. Dividing the day's checks — morning droppings, evening appetite — between family members as KinsLe Care Teams duties, each marked off with a timestamp, means nobody assumes somebody else has looked, and you have a clear record to describe to the vet if it comes to that.