agree with other fine motor skills practice as already mentioned, sewing etc, but also practice writing big letter formation on a big surface, eg poster size paper on an easel and a chunky felt tip or crayon, or a bucket of water and a brush and 'paint' on the wall outside or on the ground,
try making your own letter cards with a line of glue in the shape of the letter and then sprinkle sand over it and leave them to dry - these can be used again and again- he can trace his finger along the letter shape and then when familiar with the shape, try it with his eyes closed - can you name this letter? don't try all the letters at once, start with letters in his name, siblings names, your names, pets etc
does he know the sounds of the letters? - encourage him to say them when actually writing the letter, so he begins to connect the sound with the correct letter formation
if you are doing dot to dot gradually reduce the dots so he has to take more contrl over the shape, until he is jsut left with a starting point dot and a dot at the end of the shape - link this with groups of letters that start at the same point, eg c,o,a,d,g that all start near the top and go around to the left.
i would try and rectify this as soon as possible, before it becomes a bad habit that is harder to undo and relearn