Nicholas Lyndhurst and his wife Lucy marked a heartbreaking milestone on Tuesday - their late son Archie's 21st birthday.MORE: Nicholas Lyndhurst's wife Lucy pens moving tribute on his 60th birthdayIn honour of the occasion, Lucy took to Instagram to share a loving message for her son, along with some unseen family photos - prompting an outpouring of love and support for the couple.WATCH: Archie Lyndhurst's girlfriend and friends mark his 20th birthday days after his deathLucy poignantly wrote: