Alternatively if you bring him in after lvl 10 you could give your summoner the mount evolution and have your eidolon be small and bipedal and just give hime something like bastard sword proficiency with the multi weapon fighting feat and have one main hand and about a dozen off hand attacks all at around +9 to hit and if it takes double slice too it gets full str bonus to all the hits
i say about and around because i dont have a book on me
think about it, by level ten the eidolon can have a ring of invisibility on and you have a surprise death field next to the summoners threatend squares and you dont have to spec the summoner out for anything accept not taking an off hand ring and buying a special saddle that will fit himself
also this would require a medium pc