This is a bit like me and my.partner the other way around, although my partner also sees the need to change some of our riskier activities but just is not a worrier! So a few things that may (hopefully) help. My instinct is to keep us safe and shut us all in the house too. However, this may go on for months and I am also balancing the kids well-being and I am a bit reassured that the kids are less likely to be ill with this (don't want to leave them without a parent or grandparent but I do think I would be more anxious if kids were badly affected).
I think the other thing is to appreciate that this is a serious (unprecedented in our lifetime) situation and if your partner has done lots of reading he will understand the details more thoroughly than you. Try to have a conversation with him, that focuses on what he knows more than just on how anxious he feels. It will help you understand where his concerns are coming from and maybe help find a compromise on how you get through the next few months or however long this lasts.
On stockpiling, it is a way to feel in control and I think a good idea if kept reasonable. We may need or choose to self isolate for a while. It probably is sensible to have full not empty cupboards if you can. However, my feeling is that if it gets do bad that systems start breaking down, we will have problems beyond that able to be solved by hiding in your own house and riding it out while others sort them out. I have food for about two weeks.
On what activities to avoid and when there is no right answer. Both trying to keep a semblance of normality for the kids and trying to stay safe are valid and I think will compete with each other for a long while to come.
If you do go to the party can you have extra rules in place to minimise risk (extra hand washing, no kissing kids, no sharing food etc)? Then your partner may be reassured that others are taking his worries on board and being proactive to minimise the risk.
Can you meet friends at the park or a house instead of your normal group? So minimising risk rather than confining yourself to your house entirely?
This is more widespread in Italy than the quarantined areas (many Italian travellers are unwittingly spreading it round the globe) and people are infectious while asymptomatic. And we already have community spread that will become more widespread, possibly very rapidly.
Don't get annoyed with your partner over it if possible, everyone has different reactions to risk. Don't trivialise his concerns as they are likely valid and genuine. If he is spending too long worrying (I am) try to suggest and help him restrict when he worries perhaps (helps me sometimes) and enjoy his time with the family inbetween. Good luck!!