My autistic sons behaviour

Hi, I'm just wondering if anyone has any tips or a on my current situation , my son is 5 years old and has ASD , I'm really struggling with he's behaviour I can't find any strategies to make him understand the word NO or not to do something so for example lately he's been tipping over the plant pots in the garden and kicking the soil everywhere and when I tell him not to do it he kicks off , he smashes things, screams , throws him self on floor etc and can't cope been told no but he needs boundaries but I don't know what to do or if he won't eat he's tea he will then demand crisps or sweets and when I say no then the meltdown starts again, we are really struggling and are desperate for help, getting help from professionals is like pulling teeth. 

Many thanks