If he is a British citizen, once his DP has been cancelled, he will be given a bog standard 30 day visa. He will not be allowed to extend that visa and he will be required to leave the country for a minimum of 5 days (not published but seems to be close to the fact) The fact that he was previously here on a DP, he might be required to return to his country of origin before returning here. If he is a British citizen, he "should" be able to return and obtain a 90 day visa on entry. However, the fact that he was on a DP previously, he may well be denied an extension of that visa and he may well be told that he will have to remain out of the country at the end of that 90 day visa for a minimum of 6 months. They will already be watching that he is not using the tourist visa for residency purposes. Should that happen, he could be banned from returning.
My suggest? Don't do anything that will ruin his ability to return to the little Red Dot in the future. Forewarned is forearmed. Singapore is NOT a third world country so they don't really care where you are from and whether they are a commonwealth country or not.
That said, most love the Queen and English footy.