Pees on my mattress, just need advice!!

Hello everyone, I have a Maltipoo name Benji, Benji is almost 5 months old and doing really well overall. He’s crate trained and pretty trained to use a pee pad in the garage since we live in a very cold area and got him when it started snowing. ( he basically collapses to the ground if it’s too cold + we were waiting for his shots and then got hit with -30 Celsius weather) We plan to transition him fully outside once the weather improves.

The only issue is that he pees every time he gets on my bed. I watch him like a hawk and only let him up after he’s already gone potty, but within seconds he’ll pee; without any warning signs. Even if he just peed outside. He doesn’t do this anywhere else in the house. He’s rarely allowed on the bed but more because of this. I clean thoroughly with enzyme cleaners each time, but it keeps happening, and I’m not sure what to do.