What’s the best way to help my partner’s child handle change?

I, (29F), have been talking to this guy, let’s call him K, for the last few months. He has a child, 7M, with his ex wife (he has primary custody) and I have 2 boys (I have primary custody). We haven’t reached the stage of introducing each other to our kids. K was engaged to someone previously. That ended about a year ago (not the child’s mom). He told her he wouldn’t keep his son from her and she could see him whenever she wanted, but it was up to her. She has not seen his son since the split. That has hit his son hard. K moved with his son back to a state where he had more family/friends a few months ago. They recently moved again within the same state a few weeks ago for a better job opportunity. All of this change has been really hard on his son who has been acting out. And the thought of K starting to see someone else was not received well by his son either. K has therapy set up that is starting next week. I know that things won’t be magically fixed overnight. What can/should I do in order to help support them both with this? K feels like he’s drowning and I’m not sure what I can do to help that situation besides offer words of encouragement. I don’t know if I need to take a step back. I don’t think a video chat would help. I’m open to anything I can do to help them though. I care a lot about both of them and want to support them through this tough navigation the best way possible.