If you can, I'd try to get him back in so you can keep a close eye on him - I'm afraid one of the disadvantages of them going out is you've no idea if they're drinking or going to the loo (which can often be indications of how they're feeling).
You can check his hydration level by pinching (gently!) an inch of skin along his back and checking how quickly it goes back flat - if quick, he's hydrated, if slow, he's not.
If he's still not himself tomorrow (or gets worse), I'd be off to the vet. Fingers crossed he's fine in himself tomorrow.