PuppyMonkey/LetThereBeRock - my impression is that DH isn't allowed on here, just as he isn't allowed to speak English to their DS, despite it being his first language and that of his parents.
MmeLindt - your situation is different, though, (a) because for you, speaking German was not as a learner, and (b) one assumes your parents were not an hour or three away to be able to 'expect' your DC to speak English to them, or to be in a position where your DC might stay the weekend with their English GPs.
I loved the OP saying "the practicalities: nappies, and language". Surely the OP can see that language is being made an issue because despite her fluency and academic achievements, she has chosen not to speak English to her son. (*) Therefore, she has chosen to make a problem of PIL DC communication rather than acting in a less "precious" way to make it a non-issue.
Yes, PIL may have some shortcomings in not learning Welsh, but in day to day life, they have no need to. I have no need to either, despite living in Wales, of Welsh parents, but brought up in England (where they lived for the bulk of their lives, my mother living in Weymouth during the early part of WW II and remaining in England for more than 50 years, later living in Somerset and Sussex).
While the "dominant" language will seep in, via TV or radio, and English will be used at a later time, it seems to me that the OP would prefer to do nothing to include English at home for the next 5 years if at all possible. I suppose that means they will listen to R Cymru if the radio is on...
(laughter here) R Cymru has broadcast plenty of English words untranslated, as Welsh is unable to adapt fast enough to modern life {I have only heard snippets of R Cymru, I tend to use it only from 0100 to 0500 to have a good FM signal broadcasting FiveLive, then might switch the radio on later in the day and listen for maybe 30 seconds before switching over).
(*) That's why I made flippant remarks about DH not being allowed on MN and so forth. Her DH is learning Welsh out of love and respect for her. She knows English, even if Welsh is her preferred language, yet there is apparently quite a divide between her and her PIL, which is perhaps being exacerbated by having no apparent wish for their DS to get exposure to English in a family setting - he'll hear slang and poor English on TV/radio compared with {hopefully} good grammar from parents.