Norwegian isn't one of the hardest languages to learn, it's actually one of the easiest for a native English speaker to pick up, though it'll be even easier if you speak German. Finnish, on the other hand...
Norway is expensive, but wages are higher and social security is excellent so you get a lot back for your taxes. The university system is also a dream to work in compared with the UK - higher funding, much more humane, much less corporate, and fantastic work-life balance, though if you are a workaholic you may miss the faster pace of academic life in the UK. Having said that, if you want to make things happen then the money and the support is generally there - again, a dream after the underfunded UK.
The weather isn't great, but Oslo does pretty well compared with the rest of Norway and of course if you like skiing and winter sports then you will be ideally placed.
Is English your DS's native language? If so, he will be entitled to regular specialist tuition in it - all children are entitled to this. See above for the range of benefits you get from the high taxation. link Also, English is treated more like a second language than a foreign language in Norway so his friends are likely to catch up pretty fast as they move up the school system even thought they will be learning English from scratch. Obviously moving a sensitive child is always difficult, but I will say that the nursery (barnehage) system in Norway is excellent: well-funded, with highly-trained staff, and very focused on the children's welfare so lots of focus on inclusivity in the playground etc. Lots of outdoor activities and fresh air. School doesn't start until age 6, so he would have a couple of years to settle in, with the focus on learning through play.