Whats wrong with guinea pig?

Ive had my guinea pig for 3 years in July, he was fine when we got him but in december 2008 his little rabbit friend died, he was also okay after this but last year around christmas time he began having severe constipation.(my dog also died around this time but they wernt really that close) I kept on giving him greens such as cabbage and broccoli (NO LETTUCE) it does go away but not completley and it keeps comming back, i also feed him on hay and gr he has fresh water daily (he doesnt really drink the water despite his teeth being short) he eats all of his dinner greens and hay but he has gone so thin its scarey. He still acts like a normal guinea pig but ive noticed that when he eats he heaves? and put his paw to his mouth although as soon as this is done he goes back to munching away happily. Has anybody had any problems like this with their guinea pigs? Thankyou