Can you help with my Apple Tree?

I moved into my grandmothers house 3 years ago after she passed away and she used to garden a lot and had a couple Maple trees and 2 Apple trees. One Apple tree was cut down and the other is in the shadow of one of the Maples for a big part of the day. The tree is was apparently rescued from a compost pile about 10 years ago. It is only about 8 or 9 feet tall and 3 people holding hands could surround its branches. The trunk in only about 6 inches around. It produces about 10 or 15 small mutant red bitter apples (3 can fit in the palm of your hand). What I am hoping for with this paragraph is to see if anyone knows what I can try to help this tree, types of fertilizer, more water? It gets what water nature gives it and I pruned it last year and it has already grown back. Right now it has about 12 blossom clusters. I am in Northern Michigan (Not the U.P.) if that is helpful. Thanks for any help.

Most apples are pollinated by crab apples so its best to think of this tree as a crab apple tree and not as a regular apple tree.
So they will probably never be big or sweet seeing as how they are crab apples.

In general trees that get full sun have more energy to make more blossoms, more fruits, and the extra sunlight makes them more sweet.
So if you remove the maple then it or any other edible tree you plant will get more sunlight.

Adding fertilizer will make the fruit less sweet, and will not really affect the size of the fruit in fact it will force the plant to put less energy into fruit production and more into leave/growth.

It does sound like this crab apple would make an excellent dwarf rootstock

I doubt it will do much if it's shaded for more than a few hours a day by the maple. Either be rid of the maple, or plant another apple tree elsewhere....

If you don't know much about apples, here's a more layperson's explanation of the above.

Your seedling tree is yielding poor fruit because it probably has a lot of crabapple parentage. Trees that have a strong root system and are in a good location can be changed to a tastier variety by grafting a piece from the desired variety onto the root stock. Since your tree is also not in a great location unless you cut down the shading competitor, it might not be worth grafting onto it. To produce plenty of sweet fruit, the tree needs plenty of sunshine for photosynthesis to make the sugars.

People do grow crabapples for the spring blooms just as an ornamental, so if the tree has sentimental value you could think of it like that...

There are different uses for apples besides eating fresh. Different flavors are valued in cider-making, for instance (you can make a mini batch of cider with a juicer!). Or you could cook them in a chutney with other fruit, onions, peppers, seasonings.

Although you can't improve the flavor much of a bitter apple, I've read burying bones under an apple tree will sweeten the fruit some, as apple trees crave calcium, and calcium raises the BRIX of the fruit, which is an index of the sugars. I'm surprised it's making any fruit at all unless there's another apple tree nearby - they usually need a pollinator. If the second apple tree was JUST cut down don't be surprised if you get no apples this year - unless a neighbor has a crab apple or something.

If the maple is a tree you want to keep, you can thin the branches to let in more light or "branch up" the tree - remove the lower branches to raise the canopy to let more light in under it. If it's a Norway Maple, tho, their roots are greedy and nothing much can grow under them. It would eventually choke out your apple tree.

Renate H. : If you can give the Apple tree 5 more months you can try freezing your apples fruit, with its supposed high index of crab apple genes this may be
the secret to getting your crab-apple, apple mix to release its sugars! If this doesn't work then what you are left with is a Crap-apple! Hoping for the best! Big Al

Thanks for all the replies everybody. The Maple is over 70 years old so its going to stay and the apple tree is just far enough away that thinning the Maple wont do anything. I don't have any fresh dead bodies so burying bones is out too; looks like it's for decoration and try to use the root stock idea S. Bengi gave. There are a lot of different types of apple trees in the area (including a very big creepy knarly old one in a swamp that was here before a fire a 100 years ago that took the town) so for me the idea of identifying the apples is beyond my know how.