I just remember taking my boys outside to the park as often as possible, there was something called the "One O'clock Club" in my nearby park run by our local council, which had all manner of outside activities, sandpits, climbing frame structures, trikes, they loved it. I also took them to a lot of "Come and Play" type groups in church halls several times a week and they were a Godsend for expending energy, so they were relatively calm indoors, and if they'd had enough time outside, they were always ready for the God awful "Thomas the Tank", which they begged to watch so they could re enact all the train crash opportunities with their Brio train set. Fresh air tended to knock them out, so I was kind of guaranteed to get a daytime sleep out of them, particularly after they'd eaten. Boys do run about a lot and I think they need to get all that energy out of their system, mine seemed to spend quite a lot of time "exploding" from what I remember from the accompanying noises that went with play. It is very tiring I do sympathise, although you are a fairly young grandma but with such a condition as fibromyalgia I imagine it must be wearing for you. I also sympathise with the diet, my second son was such a faddy eater compared to my first, started spitting out food when weaning commenced, but I wouldn't have let him exist on a diet of Quavers and chips, not a lot of nutrition there and as others have stated may exacerbate the behaviour he is displaying.
I didn't use the naughty step with my sons, but I remember my granddaughter's mother asked me to use it when she had the occasional tantrum whilst I was looking after her, but she was so distraught after about ten seconds I just let her off it.
Best wishes, I hope you manage to find a way of making your little chap tired so he takes a nap during the day to give you some respite.