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?

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.

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.