Your uncle was diagnosed with stage 3 multiple myeloma? Or malignant melanoma? These are different diseases and so you should give more details if you can.
If it is a Multiple Myeloma, he has an incurable disease but nowadays there are several new treatments available which are very effective in obtaining clinical remission. Some of these therapies may cause constipation
, and so the symptoms that your uncle has, may have been caused by the treatment. For that, he should take laxatives and eventually ocasional intestinal emptyness. He should drink plenty of water and try to have a diet with high fiber content. With time it is likelly that constipation will get better.