I spent 5 years in Japan as an engineering manager. My overseas assignments (Japan and UK) were great professionally, financially, and personally. I actually found Japan to be easier than the UK (because I knew it was going to be different and I was prepared).
My advice is to go. As others have said, megacorps don't give overseas assignments to just any employee. It costs a lot of money to send someone overseas and house them. They typically invest this money in folks who they think are destined for upper management and can benefit from the experience. The fact that your son has been asked to do this tells me that his managers think well of him and are putting him on a career track that has room for advancement. This can be stressful, but can also be a lot of fun as you learn knew things and experience new challenges.
You don't say where in Japan he is going. If it is one of the big urban areas near Tokyo or Osaka, he will find that life is relatively easy but it will a bit hard to learn the language. Westerners are catered to in the big cities. There is a large foreign contingent that socializes and acts as a support network, you have easy access to Western food and other goods, and English speakers are always close by. I had a really hard time practicing my little Japanese in Osaka because every time I opened my mouth the person I was talking to wanted to practice their English!!!
The rural areas are more difficult. I had one co-worker who had been immersed for 1.5 years in a small village where he and his wife were the only non-Japanese. He had much better Japanese than I did and developed good social relationships with his co-workers. However his wife got very lonely. She was overjoyed when they moved to Osaka and she could join the expat wives clubs.
