Why peeing on the bed now?

My kitten Beaker peed on the bed for the first time last night, around 1am. He is currently on antibiotics for a URI, so I don't see how he could have a UTI. He has been with us since Dec 21, and has not had one litter box problem until now. He was 3 months old on Tuesday. I don't think it was him "spraying" per say, because it was a large amount of urine. He has always made it to the litter box before at night, so I'm at a loss as to why this would pop up all of the sudden.

Any thoughts?

It is UTI, is he declawed? Anything else change in the house to motivate this change in him? Is there another cat not letting him use the litter box? I really don't have any other suggestions, but try the vet and see what he says. Good luck!

Nope, there are no other cats in the house and he is not (nor will be) declawed.

My cats, especially Vilma, always react to slightest stress by peeing in wrong places. As frustrating as it is, I'm not worried as long as it is only sometimes.
It may be that your kitten was just stressed. If you can figure out what it was try to avoid it. But it really can be anything.
But make sure you get the smell away. If your cat can smell the pee, he will pee on the bed again. One of the good cleaners mentioned on this page is Nok-out. I haven't tried it (not sold in Finland) but it's been praised a lot.

I'd take him to the vet again. Perhaps the medication for the URI wouldn't address a UTI. Sometimes medication specifically for a UTI needs to be changed. It is very possible that this is simply a UTI that needs addressing with a different medication. I know we've used Clavamox for one and CEFA drops for the other (though I don't remember which went with which!) There are also cats with very sensitive systems, and if there's a PH imbalance, it may require further addressing through diet, etc. Tuxedo had to go on a special diet to prevent UTIs.