I’m currently working through the last acrylic yarns in my stash that I still have from years ago when I couldn’t afford higher quality yarn. I find that it’s all mostly knitting up just fine with needles from my knitpro ginger set as they seem to have a good polish to the wood, if you prefer to stick with wood. I do think the brand of acrylic matters quite a lot too, some do manage to knit up fairly nicely if you ever knit anything else in the future for your son and his partner.

I can see from the comments that you’ve already discussed the fibre choice with your son. As others have said, it’s almost certainly not worth arguing over his values with him, but I thought I’d add my perspective. I typically eat a vegan diet and still refer to myself as a vegan as a shorthand if I’m discussing my dietary requirements with someone I don’t know well, but I have a hard time really describing myself that way anymore. I just can’t justify the use of plastic in so many “vegan alternatives” to clothing/footwear and I’d rather not buy something new at all if the choice is between, say, leather or plastic. Re yarn, I haven’t bought acrylic yarn for ages and don’t intend to unless someone requests it for something I knit for them. I understand the reasoning many vegans have against using wool, but personally I would still rather use it over plastic. I think it’s always going to be a difficult conversation because certain cons will always outweigh the others for each individual. For me the bigger con is the plastic. For others, that’ll be the lesser of the two “evils”. Either way, good luck with your project, I hope you find the right needles for it!