Have you had quite warm sun? It does sound as though the Acer is getting too much sun - it could be suffering from sunburn!

They need dappled shade, and a sheltered position. You won't be able to move one that size very easily...unless you can plant something else to shield it a bit, I don't know what else you could try.

Sorry, Spritz, gonna slightly disagree with you here! This particular Acer doesn't need shelter from sun/wind particularly, except when its small (its the palmatum varieties that do), though it grows better when surrounded by other plants. It does, though, need good, moisture retentive soil - drought may be a factor in its current condition. Otherwise, inspect the tree for signs of scale infestation, or any other kind of pest or infection, things like Leaf Spot - it is also possible that the new leaves have been caught by the recent colder weather, following on from such a mild April.

I bow to your knowledge, Bamboo......:-)))