Water it well and frequently. Use a good root stimulator, such as Superthrive. Put a couple of inches of forest mulch or ground bark on the soil to keep it cool. Make sure that it wasn't planted too deep--the topmost roots should be right at soil level. I would need to know more about the exact symptoms and present culture to give more specific advice.

If its short of water it will look sad and watch out for blackfly they seem to thrive on some dry new trees?

If the "tree" you've planted is actually Solanum crispum, then it likes neutral to slightly alkaline soil which is well drained, but moist (I know, sounds ridiculous doesn't it, how can you have well drained and moist at the same time) Keep it well watered.

Mine is in clay soil Bamboo - but I do water it when dry or it sulks