Best thing is lots of large scale games to practice of letter formation. Check out school's approved style - there should be a sheet - and get him shaping letters - not words at first - with a chunky chalk on a blackboard easel, if he has one, with his finger in cheap shaving foam on the kitchen worktop, with a paint brush and poster paint on big paper or in damp sand in a sandtray. Be imaginative! A ribbon drawing-pinned on a stick is great for skywriting, like sparklers.
Group lower case letters by their shape and direction - tall ones like l, f, h, k and t, anti-clockwise ones like a, c, d, e, g, o, q, s - clockwise ones like b, p, and the loopy ones like m, n, v and w. Get his big muscles used to the directions and where to start. Lower case o starts at the top and goes anti-clockwise; it really matters! Lots of fun, lots of repetition, lots of variety.
When you go for a pencil (not a felt tip, they are harder to feel and control) get a chunky triangular one and watch his grip: a triangle of the pads of his thumb and index and the side of his middle finger is correct. If his other fingers are creeping onto the pencil, give him a cottonball to hold in his palm to keep those fingers occupied and out of the way.
Does he have lines to write on at school? Get him some widely spaced lined paper and show him what sits on the line, what goes below (the stems or loops of, j, p, q and y) and what goes above (the stems or loops of b, d, f, h, k, l, and t). Those are technically ascenders and descenders, depending whether they're going above or below the line.
Words next, and finger spaces are fine while letters are big and fingers small, but a better guide is leaving enough space for a letter 'a' between words.