As a parent of a non-verbal four year old, I understand your feelings of anxiety of so many unknowns. But that’s what they are. I’ve wished for a crystal ball many times but sometimes it’s better for your mental health to just deal with the now.
Is your son attending nursery? That can really help bring on speech and socialising skills. Also the staff have vast knowledge of milestones and can assist with knowing what is “normal”. Your health visitor or GP may be able to help with organising a speech and language therapist.
How did COVID-19 affect your family? Has your child missed out on the socialising that he would normally receive? Don’t be too hard on yourself or him, as many children have been affected detrimentally by it.
You mentioned that you have a bilingual household. That’s a wonderful thing but can slow down language acquisition in children. Does your son mix his languages?
It is possible he is regressing and losing his words but at his age there are many possibilities. Unfortunately that’s why many GPs will avoid beginning the process of ASD diagnosis until he is slightly older.