Some kids are more into toys at this age than others; my 1 year old daughter can spend hours just playing in a pile of toys, but my son at that age would bore quickly. He was more into physical activities (still is).
Also, at 15 months some kids may not have developed the skills for "pretend" and "assembly" that a lot of toys require to get the most fun out of them.
Take a look at the toy box. How long have those toys been there? Maybe the box needs a refresh! When your son isn't around, sort out all the toys he doesn't play with and put them in storage. Then bring them back in a couple months, and they'll be fresh and new (and fondly remembered). Plus it constrains the amount of junk you have to pick up when they decide to dump and scatter. ;-)
You might also try looking for "toys" around the house. Box lids, ribbons from presents, ragged t-shirts, plastic food containers, tattered magazines, an old computer keyboard... My kids spend more total hours playing with this kind of stuff than with their own toys. The nice thing is that once they grow bored with these toys, they can just be chucked.