Summer is a great time to work on this problem since there's not the added stress of daily homework assignments. Try getting your son some fun pens and markers with which he can write stories. If you and the doctor feel that his fine motor skills need strengthening, encourage your son to build with blocks and Legos, work with clay, work puzzles, and so forth. Craft stores carry scissors that cut in designs, so practicing cutting (an excellent exercise for fine motor problems) doesn't have to mean just straight lines.
Even if your doctor doesn't identify a physical problem, talk with your son's teacher at the beginning of the school year and explain the frustration you experienced last year. You may be able to work out modifications to homework such as letting your son tape record his assignments, type two assignments on the computer and write out the third, and so forth.
Also talk with the school counselor to develop a plan in case your son continues to have problems with tantrums.