The swelling is probably due to the heart murmur. A heart murmur make the heart pump less effectively, so blood tends to back up and slow the circulation down. This causes fluid to be forced out of the blood vessels and into the tissues. Commonly the abdomen fills with fluid (ascites/dropsy) and this is what causes it to swell up.
Your vet will probably have given you diuretics for your cat, to try and get rid of the fluid buildup. These drugs will make you cat urinate more (and probably drink more - so have water freely available). You will see an improvement in him as some of the fluid is cleared.
What else did your vet tell you about it?