K.,
I am a child development professional. Kids go in and out of phases with sleep so unfortunately just sleeping at 12 months doesn't insure that pattern forever but it is a great start. Once you know your child is tired then it is time to sleep. Period. Put him down and he will be mad because he is getting a personality and a will of his own. He doesn't want to separate from you or give up play.
Put him down and go in every five minutes. Pick him up and let him calm down and quietly say, "Mommy loves you, it is time to sleep". Don't say or do anything else. Just that one sentence. Do it every five minutes and I guarantee he will give up. When I did this with my son it took five times the first night. Then once the second night. It depends on how strong willed the child is.
I feel you need to go in every five minutes (watch the clock because it is a long five minutes!) to let him know you are there and also to calm him back down. But he will be very angry when you leave and that is okay.
Let me know how it works if you decide to do this. I tell parents when their kids cry picture them saying, "Mom I am so tired please help me sleep" because that is what their crying is about.
A.