Can my inheritance be withheld from me by my attorney if I still owe him $600 in fees?

While your inheritance cannot be "held back" by the probate attorney, your probate attorney can file a claim against the estate.  That is required to be paid before anything is distributed to heirs.  Depending on the terms of your engagement agreement with your probate attorney, it may well be cheaper to pay the attorney now than to collect less later and have to pay interest on overdue amounts and the cost of the claim.