score:2
If he did not visit those countries and it comes out he will likely get a ban, but if he went and 'just' does not have the stamps, but can prove it in an other way it will be fine.
I do not think the UK visa officials will be able to check with China and Singapore directly but those wanting a visa can be interviewed and in the case of missing stamps it is guarantied that the officer will ask about those visits (and likely will know a lot more about the countries and what you experience there than someone who has never been can come up with.)
And when showing up at the border (or airport) with a visa you can get interviewed again, with the same risk of being found out on a lie, and a ban for deception as result.
If it comes out he lied in the application a ban for deception is very likely and that will make traveling abroad in the future very hard.
As mentioned in the comments, the interviews for visa are not as common as I thought, but interviews on entry are still quite common.
Upvote:5
Singapore stopped stamping passports a long time back. So, you can just declare that. I have done that and no other questions was asked. Regarding china, I understand that the passport will be stamped. Was there any reason why it was not stamped?
In any case, answer truthfully and you won't be denied for deception. Include documents to provide that you entered singapore and china. For example, you will get an email just after entering Singapore, if you filled out the SG Arrival card. China may have something similar.
In any case, do not lie that you didn't enter. If they found out otherwise, they will ban you for deception.