Hello, my name is XXXXX XXXXX I am happy to help on this question.
Many Mason members have tattoos and the issue of the tattoo itself is not the issue here. As long as it is tasteful and can be covered when necessary (not on the back of hand, etc) then they do not have an issue.
There is a great site (for masons members only) but it shows some of the designs available.
Now, you are more specifically asking about a non-mason member and the thoughts on masons toward this. As long as it is done is a respectful way and in memory of his grandfather, the masons would not have a problem with this tattoo.
Having had two grandfathers myself who were masons, I would see it as a nice tribute. Personally, I would ensure that it is done in a way that is "tasteful" (which I am sure he would do) - meaning not to have it too large and prominent since it is intended as a memory of the grandfather.
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Have a wonderful rest of the week and all the best to you and your son.