What do you do when you go to him? If you are soothing and sweet or cuddle him, hug him, or get him a drink, then it is worth it for him to get up in the night.
He has to learn to put himself back to sleep so you don't have to. My pediatrician had no problem with a lock on the door. Our daughter was our strong-willed child and she had to learn that she was not going to call the shots at our house.
If there was a bad dream or illness, then we would go in and be comforting, of course. But if it was just to stay up and not go to sleep, we were stern and put her right back to sleep as a baby. As she could start to leave her room, we would just call out to her, through the slit in the door, to go back to sleep.
I would time it. Without opening the door or going in, just tell him to go back to bed. Don't yell, don't reason with him (he's too young), don't go into a long drawn out reason. Keep it to a few short words. Then let him cry for 5 min. and then tell him again to go back to bed. Use the same words as he will get bored with them. He needs to know you are still there and haven't left him, but do not open the door. Then let him cry for 10 min, increasing the time 5 min. each time. It will seem like an eternity when it is 15 min, but if all you are going to do is tell him (from a distance) to go back to bed in the same few words, he will get bored and finally realize there is no point in putting out all that effort. Your husband needs to help. It will feel like you are being tortured when you hear him cry for even 10 min, but you and your husband need to help watch the clock and hold each other back.
He needs to know you are still there. But he does not need anything else at night (on a normal night) but sleep. This is worth doing for a few nights (yes, it's difficult) but it will be so worth it. He needs his sleep as much as you do. Your new baby needs the sleep as well.
I would not suggest co-sleeping with your child if you and your husband aren't 100% excited about it. It is hard on husbands already to compete with a child, but to compete with two children and one or both in your bed could give your husband a bad message. (Already as moms, we can easily put aside our husband's needs because our children need us. Not good.) If you are both 100% in favor of the benefits, and feel they outweigh the negatives, that is the only time I would suggest that.