By any standard, that rust doesn't rate any concern. You could paint any clean rusted surface with Hammerite, if you like, but unless you're moving to the rust belt, I would leave it alone. That looks like the underside of many 13 year old trucks with 100-200K miles, and better than many.
Looks pretty normal to me.
However, if you really must have that rust free look....
POR15 i believe is the name. Just brush off the loose rust, spray on and it will bond with the rust and protect it from getting worse. Looks flat black when dry.
Kinda pricey tho... At least to me.