Now I love Scottish names, I adore
Tavish and the nn Mac so Mactavish to me would be a lovely name. Strong, masculine, yet cute. It's also a great way to honor your side of the family all at once.
I wouldn't use Mactavish and Thompson in the same combo because to me it's the same name, they both have the same meaning so it's sort of redundant in the same combo 'son of
Thomas' and '
Thomas's son'.
Mactavish
Eli would be really cute or you could name him Mac
Tavish. Thompson is also nice with like the nn
Tommy or Toppy.
Eli Mactavish would also be nice. And I'm almost liking MacTavish (this spelling) as aposed to it being all run together Mactavish.
I hope this helps a little, it's a great idea I love it! :)