Any time one of my children said they were bored I'd stop, look at them earnestly and say...
"Well, the toilet needs scrubbed, and the dog poop needs picked up out of the yard, and the laundry needs sorted and washed, and the fishtank needs cleaned....You could help me with those things, if you can't find anything else to do."
My kids learned very young to NEVER tell me they were bored! lol They're also great with imaginative play.
When faced with "yucky" chores versus "find something to do", children are really good at entertaining themselves! At 5, he's plenty old enough to learn to solo play. Provide him with the tools to entertain himself (play-doh, paper and art supplies, picture books, legos, wooden blocks, puzzles, dress up clothes, action figures, dolls, whatever), and then make it clear that Mommy isn't there solely to entertain him.
That said, with it being just the two kids, I WOULD take advantage of little sissy's naptime to spend some time reading with your son. Go to the library and let him pick out a bunch of books. Read two or three if they're picture books, or one if they're slightly longer. It's a great time to foster a love of reading! :)