Violin teacher here. Enlist them. Explain he isn't keen to practise or perhaps isn't sure how.
I happily set out a practice chart WITH my students. I ask them realistically how many times a week they can practice for 10-15 minutes. Then usually subtract a day or two, as they'll work out they COULD practise every day and tell me that, but I know that that's not going to happen, especially if they're only doing once or never at the moment!
Then I put a chart together with what days they've agreed to practise, and I write down the points I'd like them to practise on each day. So might be D major scale, and first two lines of piece X, making sure the bowing is correct, and a tune of your choice.
I try and make the targets small and easily achievable, and end with something they find fun. Practising scales to silly rhythms often works for example as they like making up sentences to play and don't really realise they're practising their scales at the same time!
I also include a section which says 'what I think went well' and 'what frustrated or challenged me' for the student to fill out each time, because actually they often forget what they've found tricky during the week and it means my lesson can be more focused to support them with that. It's also really important for them to recognise what went well- and really small stuff counts. 'I managed to play bar 1 perfectly' is GREAT. They need to be able to identify what is improving to feel the practise is worth doing.
Other techniques; find somewhere in the house where the violin can sit undisturbed but with the case open and with the instrument ready to play. A closed case or an instrument tucked away is a surprisingly big barrier for some children, and you might be surprised at how often he just picks it up to have a go at something during the week if it's just there looking at him.
I often ask them to compose their own little tunes to play to me, or to work out something by ear. Christmas coming up is a super time for this; can you work out how to play the chorus of Jingle Bells? It starts on an F#. They really get a sense of achievement from this!
And finally, I ALWAYS tell my students' parents that I don't want playing or practising to be a battleground at home- you have enough to do with homework etc. Set a time and remind them, yes of course. But argue with them about it? Nope. It's my job to give them the tools and the enthusiasm to WANT to practise. A forced practice with an unwilling child isn't going to gain much anyway.
Good luck!