It's a decade ago now but I was a Young Conservative interviewing in a pretty left wing industry, and I always found honesty was the best policy. I had my voluntary positions on my CV and was always open about where my experience came from at interview.
Politics is not for the workplace so I often would acknowledge that directly, then demonstrate it by not talking about ideology at all and instead focussing on the skills my roles helped me to develop.
Getting a question about a difficult situation can also be a really good one to answer with political experience - I bet your son has hard conversations on the door step. He should use his answer on that as an opportunity to demonstrate that he is reasonable and not an ideologue (e.g. by saying he can understand why people are angry with the government, and so while it's hard to be shouted at he keeps in mind that people are struggling and stays calm, tries to be kind, etc).
He should also make sure that he talks about other experience/interests so that he isn't presenting as one dimensional.
My approach was successful. I know times are different now, but I reckon the Tory hate in a left wing industry 10 years ago is probably roughly equal to the Tory hate in finance nowadays 😂
I think his experience is useful and if he tries doing as PP have said and omits what party, it'll be weird. It may well make it more of 'a thing' than mentioning it once then focussing on skills - if he doesn't say, they'll probs be listening and trying to figure it out, then wondering why he didn't say, then maybe feeling a bit deceived.