Won't stand up

Is there something he would hold on to? I’m thinking maybe a toy that you could both hold. Alternatively, would he stay close to you (via verbal instructions) without physical contact?

Some of the advice on this NAS page may help:

https://www.autism.org.uk/about/behaviour/guidelines.aspx 

Sorry I can’t think of any more suggestions. 

I should of said in the beginning he is diagnosed 

If he was doing this "dropping down and lolling behaviors" for no apparent reason, ie it is behavior he cannot control then although it may be seeming to you he is doing it on purpose, something may trigger it, environment, noise etc then as I mentioned earlier this could well be indicative of an autistic behavior,  others may advise further. if they have had other experiences.

On a practicable way forward keep a diary of when this happens and the circumstances over a period of time, maybe a month. I recorded happenings for about a week. When my son used to drop down for no apparent reason other people would make sarcastic comments in supermarkets etc. with your child lying on the floor and refusing to get up. After a while we would retort "You are  lucky not to have an autistic child". and ignore them, and allowing the child to lay there until he wanted to get up. This was important in establishing a physical disability in order to get a handicap badge for a physical disability, which was an important help in transporting an autistic child. otherwise it meant having to strap the child in a pushchair, just to transport him from any parking space to shops. 

When It it happens in an out door situation, a photo may help explaining  your problem to others. .

If you put his reins on under a coat and pass a strap down his arm for you to hold his hand then it would look more normal than if you put a wrist strap on your son, some might object to that if you tried to lift him by it. If he does down, record it soon after wards when the panic is over, and leave him down unless he is in a difficult position, puddle, rain etc. He will get bigger and heavier. one day he will stop altogether. With just a few records go and see your G.P. and request an expert examination of him for being on the autistic spectrum.

As before all the best for your family in coping, 

Thank you for your concern , but he is not tired he is doing this on purpose  when out and about not all the time I just wondered if any body else has had this problem and had any advice as reins are no good will try wrist strap today 

My advice is to take him to a hospital ASAP if this has recently happened. take some coins for a vending machine or something to snack and drink.  IIt could be he is very tired. and wants to sleep. You will know if this is new or he has been like that for 5 years, Is he taking any medication, or has he found anything toxic, Is there any child, or others  who could have fed him anything, could he have fallen and hit his head unknown to you. He could have caught an illness.

If it was me I would take him to the Hospital now, even at this time of night. There are so many things to cause his problem some very fast acting.. 

My autistic son at an early age was addicted to salt, We have had to hide salt except for very small quantities for 30 years. There are many things a child could get at.  He also would lie down when out and scream and object,if you tried to pick him up. You seem to indicate the lolling of his head and body 

If you are really worried as I suspect you must be. If you have no transport dial 999 now and ask for ambulance. It is better to believe you may look foolish if there is nothing wrong., I assure you nobody will think you are over protective. that will not be the case.

Best of luck and hope for your son.