Year and ten month old male rat suddenly struggling to walk

As the title suggests, one of our boys is suddenly struggling to walk and isn't as lively as usual. When he comes out to eat and go to the litter tray he seems to roll backwards onto his back and sit like that. Is this just HLD or something more serious we should go to the vet about? We had to have one of ours PTS on Christmas Eve, so a bit worried about the ones we have left. Thanks