It'll be alright in a large pot for a while, but its roots may be widespread by now and therefore difficult to fit into a pot. This variety of Acer grows quite slowly and forms a mound of foliage which is usually much wider than it is tall, so it needs quite a lot of space around it.

Lovely wee Acer but not going to take kindly to a pot after having the run of the ground. As Bamboo says the root structure is going to be larger than you probably think

Done right now it might go into shock and have a major leaf drop. When would be the best time to move it?

Mid autumn if Karenbro really must, but given the diameter of the canopy I still wonder how well it will survive. Karenbro this particular acer isn't going to grow tall it is one of the low, weeping type ones.

Oh thank you all for your advice! maybe I will leave him where he is. He's a sweet little guy and I don't want to stress it out by moving.

Surely its ok to plant something near him though that doesnt root too deeply? like some meadow flowers etc?

What do you think.

No problem planting near the acer, we have several like this and so long as you leave about a foot around the trunk you can plant bulbs or anything else that isn't going to grow too tall. Remember though the leaf canopy will be just about down to the ground.

I planted one in my daughters garden over 15 years ago. To keep it to the space she wanted filled I just trim it back.