L., he does have you trained! I think that you need to let him learn how to soothe himself back to sleep. Do you do the bath routine in the middle of the night? The reason I ask that is I tried taking my granddaughter out of bed and doing something to soothe her. It didn't work.
He does not have a sleep disorder. Nearly all babies go through this. At 13 months he doesn't need the nutrition of milk during the night. If sucking calms him or if he gets thirsty during the night, I think it would be good to give him a sippy cup with water when you put him to bed. Both my 6 yo granddaughter and my 3 yo grandson frequently take a cup to bed with them and always have. I've seen them take a drink without even waking up.
I had a difficult time letting my grandchildren cry themselves back to sleep but they stopped crying, only fussed some, in a short time after I stopped getting up with them. They would sleep thru the night with their mother but not with me.
I think one should start by going in to them, tuck them in again and leave. That reassures them that you are there. The books say don't talk to them. I did, in a soothing voice. You could also just sit in the room with them without talking. I think I did that some of the time. Whatever you do it has to be consistent and soothing while leaving them in bed. You want him to learn that in bed is where he belongs.
One book that I read suggested letting them fuss a little longer each time before you go to them. ie; start by immediately going in then in a couple of days wait 5 minutes and then 10 and so on until you're not going in at all. That required too much thinking for me and I didn't have them consistently for several days in a row.
It is very hard to hear them cry and I also believe in not letting them cry for very long. If they keep crying until they are in a frenzy I don't think that they can learn how to self soothe. they will eventually fall asleep tho if you can stand it and that works for some parents.
I remember that, as an older child and then as an adult, when I was especially upset I did cry myself to sleep and for me crying in itself was soothing. And so I am confused about how long to let them cry. I think fussing and some crying is OK because it is soothing but hysterics are not.
I do know from experience that if you always go in and fuss over them it will take forever for them to learn how to get themselves back to sleep.
One of my favored expressions is: this too will pass. One advantage of being older is that I now have the years of experience to know that is true.