Which bank credit card and application strategy will you recommend to a 29-year-old male working professional with no prior credit card ever, just a DBS account and debit card?

It depends on the type of reward that he wants, whether is it cash rebates or miles. You can make use of online comparison sites such as MoneySmart and BankBazzar to compare which card suits him in terms of rewards, amount/percentage of cashback, minimum spending (if any) etc.

Usually you'll be able to get some sign up gifts or deals such as getting cash credit for spending a certain amount within the first few months of your card approval.

Ask him to use this calculator to see which high interest savings account is the most suitable for him.
https://seedly.sg/tools/savings-account-calculator