15-y/o male cat with bowel issues & occasional vomiting

We have a jet black male 15 y/o cat that has occasional bouts of vomiting & diarrhea. He has lost some weight but not enough to be a concern, his coat is glossy, & he hasn't been lethargic. He yowls more, especially when deciding to go to the litterbox. But he purrs when we cuddle with him & appears to be generally content. He scoots on the carpet often after visiting the litterbox, too, & has left droppings as he dashes out of the litter box sometimes. He's always had a "nervous stomach" all his life (he can be pretty spastic) & has a tendency to eat mouthfuls of food rather than a small bites. He's always afraid he'll be run off from his food bowl by our "alpha cat". He'll go a few weeks without vomiting or diarrhea, but then it will start up for several days. His eyes are alert & he doesn't display any signs of discomfort or pain other than the occasional yowl as he goes into the litterbox. His appetite has never been voracious but he eats regularly (when he doesn't get nudged out from the other cat). He's got some Siamese but do not know of any other breed. We're not sure if we should take him to a vet or just change his food. Any thoughts?