Cat is sleeping more than usual

My kitten just had a vaccine to prevent parvo 4 days ago. Today (4th day) my kitten ate in the morning but tonight he does not eat yet. This is not his usual behavior and he keeps on sleeping and sleeping. Even though I try to wake him- he won't budge and will just sleep beside my husband. He is usually playful and eats a lot for a 4 month old kitten. It's been raining a lot and cold. We texted his vet now (it's currently mid night). But am wondering if he's fine?

His brother cat (not his biological brother but they're close-knit) just died 5 days ago of parvo. :(

They were separated at start of brother cat's sickness. Totally isolated from one another.

Please let me know what you think. Thank you!!!!