Autistic son hurting Dad in meltdowns

During meltdowns my 6 year old will always focus all his anger and aggression on his Dad. He will go out of his way to try and find him in the house or follow him around the house (if his Dad tries to move away) to carry on hitting him. The advice we have been given is to keep him in a safe place during meltdowns but he won't stay in one place, he just wants to be hitting his Dad. So far the various professionals that I have spoken to about aggressive meltdowns have said all we can do during an aggressive meltdown is wait for him to calm down. One professional suggested my husband holds a cushion to protect himself (but that seems to make my son even angrier and he'll just try to move around to hit his Dad on a side not protected by the cushion.). My mother in law suggested that (if I am home) my husband should lock himself out in the garden during aggressive meltdowns so that our son can't get to him. When he tried this, our son was then thumping on the door trying to get to his Dad and I was worried he was going to hurt himself.

I am worried about both my son and his Dad. Mentally it is very draining and stressful but it is obviously really upsetting for my husband to be constantly be on the end of such aggression from our son. One professional said eventually as our son gets older, he will grow out of this aggression but then other information packs have said that if the aggression isn't treated at a young age it will get worse as they get older. Outside of his meltdowns he is the most loving child and is really affectionate to both of us. 

Has anyone experienced this and does anyone have any advice? I have tried speaking to a GP once about his aggressive meltdowns but was basically told that they wouldn't do anything about behaviour at home, only any behaviours that occurred at school that were affecting his education.