If your son remains a "resident" of the UK he may well have to. Some countries do have a "double taxation agreement with the UK" which means he may well not have to pay tax in both countries.
Have a read of this link, it provides most of the information he may need.
He isn't currently a taxpayer, being a postgraduate student. This is quite a long reasonably well paid placement in a none EU country. I presume he will be taxed there, and it would feel unfair if then pays income tax a second time.
Presumably if he never tells HMRC, and they don't ask, he might slip under the radar
Oh jeez hopkirk that could land him in a whole heap of trouble.
If this is the start of his working career then he needs to do it right otherwise this could come back to haunt him later - and HMRC always catch up with you sooner or later.
There are penalties for doing what you are suggesting.
He's only just been offered the placement. It's a highly reputable company, and I'm sure many others have passed through this before, so I expect they will give him good advice.
I suggest looking at the HMRC site.
If he decides not to declare it and then tries to bring money into the UK at a later date the banks will notify HMRC.
If he comes backs and works in the UK there will be a gap in NI contributions questions will be raised as he hasn't signed on.
I could go further - best case scenario if he is paying tax there he won't have to pay it here - worst case he will still have to pay some here - wouldn't it be better to be sure from the start.