Regressive Autism

Our son had been developing normally until about 18 months. Slow to develop verbally but had 20 or so functional words and hitting milestones with receptive language(put on your shoes, point to your nose) good eye contact etc..

Around 18 months these skills started to erode and new behaviors started to show up (walking on tippy toes, lining up toy cars, feeling objects for texture). We started him with SLP and OT at 24 months and we are on a year long wait list for an official diagnosis.

Has anyone else had their little one hit most milestones only to have a severe regression between 18-24 months?