I play Pokémon with my sons as well, with the same general strategy - the older one (7) plays Sun I play Moon (and the younger plays whatever he wants, he's only 5).
The way I approached this, particularly when we were starting out (at 5), was to partition my help into different "buckets" mentally.
- Help using the interface: I would show him how to use the interface by doing the action on my device, expecting him to then do the same on his.
- Help finding a particular person/pokemon/item: I'm the sort of person who happily consults internet sites to find out where things are, as I don't get a lot of enjoyment from the "stumble upon" style of gameplay. As such, I treated interactions with him as similar: if he asked where something was, I showed him the process of looking it up on the internet. (We generally let our kids at this age look things up freely, given we're constantly supervising them to notice anything they shouldn't be doing.)
- Help solving a puzzle or working out a difficult solution: Here is where it gets harder. Most often, what I do here is let him struggle a bit, but keep him from struggling so much that he gives up. It's a hard balance. Mostly I ask him questions, rather than giving him answers, trying to get him to think about what he's doing; that works well with my seven year old but less well with my five year old.
For the most part, though, what worked best was letting him go at his own pace, and me going at mine, and just talking about what we were doing. With my 7 year old, I tend to be more 'exploratory' while he tends to be more "direct", which works well because it means we finished at more or less the same time. It lets him focus on the stuff he enjoys while still letting us do similar things.
With my 5 year old, it doesn't work as well because his attention span is lesser. That means he constantly asks how to get to the next place; I largely don't tell him, other than to remind him what the game text told him, and to show him the Rotom Dex. If you're playing Red and Blue or something older, this will be more troublesome; but if you're playing Sun and Moon or US&UM, the Rotom Dex pretty much tells you where to go, so you don't have to focus too much on that. If that's the part that's hard for your seven year old, you might consider going to a newer game (X&Y or newer, and particularly S&M or newer).
One other thing of note: I did trade him a few higher level Pokémon in the first playthrough (Pokémon that were just at the level cap for him at that stage in the game). That helped the combat portion be a bit easier for him, while still requiring him to actually do the combat.
We also regularly talked about what we did and why; this helped him without actually making him dependent on me, because it let me explain strategies I used (such as using particular non-damage moves that helped make battles eaiser). It also helped that he was hugely into Pokémon before we started playing, so he knew the basic matchups and things like that (fire beats grass, etc.) We also regularly battled each other, which he quickly got to the point that he could beat me regularly in, but definitely helped him learn strategies better since he couldn't just brute force his way through an even-level battle against a variety of Pokémon.