1) A wild boy with superhuman physical ability who lives with an old man in an isolated place
Conan is a wild boy who lives in an island named "Nokosare Jima" (remaining/left island). In the island, Conan and an old man are the only survivors after the worldwide disaster caused by mass-destruction weapons. He has nearly superhuman power and is even able to carry a dozen of mature people. Goku lives in a remote mountain with an old man. He is so strong that he can carry a car.
2) The boy calls the old man grandpa, but they're not actually related
Conan is an orphan whose parents were also the survivors who once lived in the island before. They both died for some reason and he was raised by the old man. Conan calls him Ojii (grandpa) like how Goku calls Gohan Jii-chan. As you know, Goku is a Saiyan who was sent to the Earth and not related to Gohan.
3) He meets a girl for the first time while carrying a gigantic fish that he caught
Conan catches a shark in the sea, and on his way home, he finds a girl named Lana who drifted to the island. Goku catches a gigantic fish in a river and meets Bulma. This is the first time ever in their life to meet a female human for both of them, and the event becomes a springboard for a big adventure. It's also interesting that Bulma's name was Lena in the Harmony Gold dub though this must be completely coincidental.
4) Yajirobe and Jimsy share multiple characteristics
Jimsy is one of Conan's friends who also has very high physical ability. He is a little plump and a very big and adventurous eater who regularly eats rats and frogs. He lives alone by hunt, and nothing about his identity or family is told. Not only these characteristics are similar to Yajirobe's, their appearances are somewhat close to each other.
5) Yajirobe/Jimsy fights with the protagonist because of stealing his meal in their first encounter
After leaving Nokosare Jima, Conan arrives at the island where Jimsy lives. He eats some fruits and drinks water from hunger, but he didn't know he's in Jimsy's territory. Jimsy gets angry at him stealing his meal. They fight and become friends. This almost completely corresponds to how Goku meets Yajirobe for the first time.
So, has Toriyama ever seen Future Boy Conan?
YES, HE HAS. After noticing these similarities, I searched for the relations between Toriyama and Future Boy Conan. I found his comment in Shonen Jump vol.3 in 1985:
Toriyama wrote:今頃になってかの名作『未来少年コナン』を見て大感激しちゃいました。ああいう作品をかきたい<明>
It's not clear whether it was his very first time to be exposed to the series or he'd already seen it partly before the serialization, so I'm not still 100% sure about the similarities in the first chapter, but Yajirobe must be based on Jimsy at least.Recently, I finally watched that masterpiece Future Boy Conan, and it really impressed me. I want to make something like that. <Akira>
