Kids aged 4 ish will often write in non standard ways. My ds always wrote backwards and round the corner!! They soon learn how to do it "correctly" and schools work with them doing handwriting practice. Don't worry and don't pressure just let him watch when you do any writing so they can see how it's done :-) be happy he's forming shapes that are accurate and that he's enjoying it!!
My DS is four and loves writing numbers and letters but without fail starts at the bottom right-hand corner of the letter/number and works his way up. I've tried gently suggesting to him that it goes the other way and trying him with sheets with the arrows on it but he insists on doing it his way.
I don't want to make a big deal of it because it's great he loves writing but is this the sort of thing that corrects itself?