J., there's no way of getting him to nap for longer periods of time. As babies and toddlers grow, their needs for naps change. The naps get shorter, and they eventually eliminate one nap. Your son is doing that now; it's perfectly natural.
Why do you want him to sleep more? If you need more time to yourself, then you can try getting a babysitter, maybe an older child, to give him attention at a certain time of the day while you do whatever it is that you need to do. We used to call these older kids "mother's helpers" or "mother's friend." I'm sure there is a girl or boy who would come and play with your son for a couple of hours a day, if you give the kid a couple of dollars.
Alternatively, you can also find someone who would be willing to mop your floor or whatever for you, while you spend time with your child....
It might be time for you to get a playpen so that your boy can be in the same room with you but not in your arms all the time. Some kids do well with this partial attention, but some don't. Also a walker or a swing can keep the little one occupied, playing by himself. He might be ready for something like that.
But please, please, don't try to force him to sleep more than he needs. He's going to feel rejected, and this is going to cause a world of hurt for him and the whole family, now and in the long term.
I hope you find a solution that will keep everyone happy and feeling blessed.
Peace,
Syl