Thanks all. This is worrying. Its 2 am currently and no vet would be available. But will certainly take him first thing in the morning to the vet. He is thankfully sleeping soundly and I am watching over him.
I feed him chicken liver, rice cooked with some vegetables in the morning and Royal Canine in the evening together with rice, some curd/yoghurt. He is a great, non-fussy eater. However, just yesterday shifted from Royal Canine Maxi Junior to Royal Canine Adult. Not sure if this might be the problem since my second one, a basset hound, who eats the same food, has been fine.