Autistic child activley seeking to hurt parents during meltdowns-any advice please?

My 5 year old son has suspected Autism but won't start the assessment process until the Autumn. We've been doing as much research as we can and are trying our efforts to prevent/respond better to meltdowns. Now on week 2 of the school, we are on daily meltdowns. 

He gets aggressive and really angry during meltdowns and we have tried following the advice of letting him have space for his meltdown and leaving him until he has calmed down  or advising him to go to his quiet tent or giving him a squishy toy to play with. But what do you do when you try to leave the room and he runs after you trying to hit you if you are within reach? If one parent say goes upstairs if he has been hitting then, he will then go and find the other parent and start hitting them instead? 

We try to hold his hands and explain calmly that we need kind hands or simply "you do not hit" and move away from him. But short of locking ourselves in the bathroom while he tries to break down the door to get at us, I don't know how you are meant to calm down a child who is so determined to hurt you, while keeping him and you safe? 

We luckily don't have any other children and he is only 5 but the older he gets, the more he can hurt us and we worry about him being aggressive to other children if he was to have an aggressive meltdowns at school (he usually masks at school so they rarely see this side of him) or if he tries to hurt his Grandparent's who look after him during the holidays.

The rest of the time he is the most loving and affectionate child but we are really struggling with his aggressive meltdowns. 

Thanks if anyone can give me any tips!