You are so lucky to have had a great sleeper. This could be a rough phase if he has teeth pushing through. Also make sure that as temperatures are cooling that he's not getting cold at night. Or, as others have said, you may need to increase the amount you're feeding. If he's pulling up then he's getting more active and will be quickly burning off his meals. A bowl of warm oatmeal before bed helped my daughter sleep longer.
If it's just a new habit he's developed then I went through he same thing. As soon as my daughter woke up she would imeadiately pull herself up and stand there crying, I'd lay her down, she'd pop right back up, ultimately I'd cave in and pick her up and rock her back to sleep. I let this go on for months thinking it was simply the cost of putting my daughter to bed without leaving her to cry it out.
Finally one night I went in exhausted and just sat down on the floor with my head down by her mattress. I didn't make eye contact with her standing over me and it didn't take long before she layed herself down to be able to see me. This was an "AH-HA" moment for me, of course she's always going to jump up since she wants me and I had always loomed above her.
From that point on, when she woke I'd just go sit on the floor next to her crib and after a couple of nights she stopped standing up, instead would only sit up, and by a week she was putting herself back to sleep staying down completely. Within two weeks we had gone from waking six times a night to only one. Hope this helps, good luck!