My son cut himself with a knife it was pretty deep Er Dr applied Dermabond to it and said no tetanus since last once was 5 to 6 yrs ago, is this ok?

Yes. a cut from a kitchen knife is not considered a "dirty" wound that would require tetanus booster. within 10 years is good for a clean wound. dirty wound like stepping on rusty nail would require booster if more than 5 years since last tetanus.
5-10 year range. If the knife was a rusty garden tool, most use the older "5" year recommendation for booster. If clean (like stainless steel kitchen knife), tetanus not highly likely and current recommendation is every 10 years.