The DS9 Reference Manual, p42, 'Numeric Formats'. http://ds9.si.edu/doc/ref/ref.pdf#page48 has a bunch of information.
Based on that, I expected that "tlhm.1" would achieve what you proposed.
However, it seems that SAOImage DS9 is very keen on dropping leading hours it sees as unnecessary, even if you have specified them in a format specifier.
I haven't been able to figure out a format specifier combination that retains the hours for every gridline while also showing the minutes or seconds.
I have searched elsewhere for advice on this and haven't found it either. In fact, I haven't been able to find any comment on the hours being suppressed, nothing that explains why it does this, nor even anyone else remarking or complaining about it.
I suggest sending an email to the DS9 Help Desk address found in the menu under Help > Help Desk.