When I stopped nursing my youngest son and weeaned him off of co-sleeping he was younger than your partners son. I took his matress from his toddler bed and placed it beside mine on the floor, then every night I moved it further out of the room and down the hall eventually to his own bed. It actually only took a few weeks. Then after that if he woke I would go calm him and lay with him and rub his head for a few minutes (in his own bed) and he would be ok. Which did not go on for too long. Now he never wakes at night. Maybe you could try something like that.

A lot of experts also advise of the benefits of co-sleeping, so there ya go. Experts never agree. But it can help create a strong bond. However at somepoint he does need to learn to sleep on his own. I remember I had really bad night terrors when I was a kid and would wake my mom or wake her up with my screaming untill I was about 12. Maybe there is a reason why he is having such dissrupted sleep? Maybe not but can't hurt to try to find out why he is waking so much and seems scared. Maybe you and mom can work on illimanting his fears or whatever it is.

But be patient, it will get better if you both keep trying to get him out of this routine. It is a pretty normal stage they go through in these young years, even though is can be very frustrating to the parents sometimes.