insurance bond /banker guarantee is not sold by Insurance co.
go to your banker, they will ask u to pledge the same amount in cash + their commission.
if IRAS ask for S$50k, then put S$50k in the bank.
there is a letter to be issued by the bank, but IRAS must agree the term & condition.
get the draft copy from bank and send to IRAS to confirm their acceptance.
the T&C would include how this bond will be discharged eg upon his return, and validity of the bond (this bond cannot go on forever, if IRAS will no action within X days, the bond should discharge itself), etc.
talk to Commercial section of your bank.
if he don't return, the bank will forward the $ to IRAS automatically, the bond also considered discharged. (ie the bank's job is done).