Whenever kids hit a milestone, their sleep is disrupted. After kids turn 1, they will go through multiple sleep disruptions even if they were good sleepers because they go through so many milestones. It's very temporary. It happened to my daughter, too, after she turned 1. She would fall asleep fine when I put her down after the bedtime routine, but she would wake up in the middle of the night, sometimes several times a night. Most notable was when she was learning to walk. Generally, it seemed like the disruptions seemed like they were going to last forever when I was actually living through it, but after they were over, I realized they ended up lasting about 2 weeks at most.
Regarding nursing him back to sleep, I wouldn't do it if he was fine all this time without it. It'll become a habit. Just try to soothe him a little and put him down. If you haven't already, get the book Touchpoints by T. Berry Brazelton, M.D. He talks about going through milestones and how it disrupts sleep and what to do.
Good luck and hang in there!