Could just be a reaction to the shots but watch him close and I would make sure he had something liquid in him until he perks up or you take him back to the vet.

When I first got Samantha, I took her streight to the vet and got her shots started and I was sure I had killed her in having her get them in the horriable condition she was in when I fianlly got her. She laid in the kennel for 2 days, not moving, groaned when you tried to touch her and felt like she was burning up with fever. I called the vet early on day 2 and they told me to give her the night and if she was no better to bring her back in. Next day, she was up, eating and her old self. She was half starved and eaten up with fleas, skinny as a rail and dehydrated when my neighbor finally agreed to let me have her, a year ago last July. I put off having her spayed until she was a lot stronger. Now she is filled out, fattened up (still not fat), grown a beautiful shinny, silky coat of fur and healthy as any well loved, pampered kitty can be.