Crab Apple Tree.

My son bought a crab apple tree 10 years ago. It produced small round green/rosy apples. After a few years it was producing 2 types of apples.  Small small round green/rosy apples and larger darker green apples.  It now only produces the larger darker green apple.  Can you tell me if these apples are still crab apples.

Think we need some pictures of the tree and the fruit to try to fathom out what's going on 

Sounds like the rootstock has taken over from the named Crab apple type to me.

Agree re root stock.  If it was a grafted tree, and most of them are - then track the new shoots down to the base and remove them as far below the soil as you can.  Keep an eye on the base & remove any suckers you see coming out from there in the future.  If it was not a grafted tree, then I am at a loss .........

That's why a photo would be helpful - but I would have thought that a crab apple would have been grafted onto m. sieboldii or m. robusta, (that's my understanding, but I'm not an expert)  neither of which have large green fruit.  

I'm wondering if it might not be a crab apple, but perhaps a grafted 'family' tree.

Perhaps, but the rootstock under one of our Crab apples (M. trilobata) I took and grew on.The apples on that one could describe as sizable and green. Say 2 inches in diameter?

Many thanks for all your suggestions.  When it was producing the two different apples they were on different branches and different sides of the tree. . It was as if it was two different trees .

It does sound to me as if it was two different types grafted onto one rootstock - like a family tree - possibly an attempt to ensure pollination 

http://www.thompson-morgan.com/fruit/fruit-trees/apple-and-pear-trees/family-apple-tree/dww3680TM?source=google&gclid=CKH34ZSLlbICFcQNfAod-hkABA

I've come across instances when only two of the three grafts have taken, and so a tree with two varieties grafted onto it get sold off more cheaply.

crab apples come in many varieties .the wild ones you pick out in the wild are small and best for jelly.You can get some that are different shades of green and red which gives the jelly a lovely colour.I have a tree which was a cox's until it was damaged and then it reverted to being a crab.