Sorry for your loss. I was in similar situation few years back. According to our customs, you should follow those restrictions at least 5 days since your a grand son. In my case, I did for 9 days as a respect to my grandmother. Dietary restrictions means you do not consume salt, no any meat products and no onions & garlic. Also, if possible you do not want to eat outside during these days and try to make your own food. I pretty much ate fruits, milk, rice, karela, saag and some ghee during that period.

It sounds tough but it is doable. I would advise you to go ahead and do at least five days. If you feel you can do more, you can carry out till 7 or 9 days. When it gets tough, just keep in mind the things your grandfather did for you and this is the last thing you will ever do for him.

R.I.P