I've never heard any medical research or advice that might suggest prolonged periods in the swing could be harmful.. but I do wonder how you've been able to keep your child in the swing at a year old! Both my kids had outgrown the swing by 8 months or so!
In some of the sleep books I've read, most suggest NOT putting your child to sleep by riding in the car, rocking, or by swinging because they create a "sleep crutch" that usually needs to be continued throughout the nap or night.
However, now that I've had a poor sleeper for 2 years, I say "Whatever works!" Maybe you can try to transfer him earlier? Or trying some other movement to get him to sleep, like rocking or walking around with him, or pushing him in a stroller around the house?
GOOD LUCK!