Acer palmatum

Thanks for your answers. So even if after the cut of the airlayer the nebari will be flat, should I use a deep pot for the growth of the tree?

A flat nebari is something you create and maintain. It just do not grow a good nebari by itself.

A big pot. Possibly plant it in the ground after established in the pot.

Yes, might worth mentioning. If you see the first roots now, it may take several months more before you have a root system that can support that tree. At least in my conditions.

I would strongly advice you to be patient. The more roots they grow, the better the chanse of success.

Thanks for your answers. It is difficult to understand how is the root system now, because I have done the airlayer directly in the ground. Anyway I did it at the end of May and I will wait for September before removing the ground and cutting it. Should it be enough? 

I would wait until repotting season, so March or so, depending on your climate. 

After a period of good growth, the leaves of my maple started to become dark as in the picture.

 

The tree takes only 2 hours of direct sun in the morning and I keep it moisty. Unfortunately the soil from the nursery was quite bad and the drainage is not great. Anyway for the first couple of months I did not have this problem that only started at the beginning of August.

What do you think can be the problem?

It is a living plant. Leaves die. The weather this summer has bee hard on all plants, and a couple of hours getting dry can damage the leaves. I would remove the damaged leaves and leave it at that.

Here in Alabama a lot of trees have a semi dormant couple of weeks mid August and lose  leaves. The cherry tree in our front yard has dropped a lot of yellow leaves all over the yard, as has both dogwoods. Some of my bonsais have done the same. 

If you are in a hot area this is probably perfectly fine.