I wouldn't tell him, given he's looking after DS. It's not about him having a good time on holiday (he's already worried and on edge, so it's already spoilt). It's about making sure he doesn't accidentally or intentionally do anything stupid to endanger him or your child.
When I found out my parent was dying, I acted very calmly on the phone. That was perfectly in character: I'm the glue. I'm the one who always copes. Then, as soon as I hung up, I walked in circles for hours. It felt like the sky was falling in, and I had no idea what to do or where to go or how to react, so I just walked and walked and walked in giant loops, sobbing like a crazy woman. I'm sure people looked at me like I was off my rocker, but I couldn't see anyone or hear anyone, I was lost in my own spiral of misery, walking and walking.
I'm not sure what I was trying to do. I think that was the problem. I couldn't figure out what to do, so once I'd settled on something (walking), I couldn't take that thought any further, and without a planned destination, I just kept walking in circles. I suppose the issue was, as soon as I stopped, I'd have to go back to processing the news and dealing with it.
I'm actually crying now just at the memory. That's how painful it was finding out.
I'd say there's no good way to tell someone. It's shit news. So whatever you do, don't beat yourself up if it feels like you've got it wrong.
However, I would strongly caution against telling someone when they have no in-person adult support around them and when they are supposed to be busy being responsible for someone or something. I'm surprised at how badly I reacted, but I guess there's no playbook for what to do when you find out your parent is dying.
I'm not saying I would have deliberately harmed myself, but I could have easily had an accident given I couldn't really see or hear through the overwhelming grief. As your DH is solely in charge of your DS at the moment, I don't think it's right to tell him. Wait until they're both back, and you can hold your DH after you tell him. When you are in so much pain, human touch is the only thing that can take any of the hurt off.
In the end, a friend came to get me. I don't know how I would have made my own way home without her. Rationally, I knew where it was, I was just caught in a loop and couldn't make myself break it, go back to the reality, and go home.