The best thing you can do is to leave it alone. Trimming a weeping tree can destroy the shape and defeat the object of having it in the first place. Fertilising a tree is unnecessary unless there is a proven nutritional problem. Crab apples can grow in a wide range of soils.

Just keep an eye on it. Don't panic over a few chewed leaves. Pick off caterpillars or blast off with the hose. Allow birds to deal with them too. Water if it gets thirsty. Otherwise let it grow in peace. It's a tree, not a person so anthropomorphising it (it's not a 'him') could lead to 'loving it to death'.

need a pic.. if you want opinions ... ken