Yes, this is really harmful to his eyesight. Reading by candlelight can cause eye strain through making the eyes to adjust constantly under the flickering light. Eye train is the main cause of many eye disease such as myopia, dry eyes, astigmatism and so on. Homework is important, but not as valuable as the eyesight of your son. Myopia and many eye diseases are irreversible. Think about it!
Yes, reading by candlelight in the evening will damage his vision. Not suitable amount of the light will cause the nervous state of the eyes nerves. He will squint to see the words. If he does this for a long time, your son may easily get short sighted. It is not a good habit. You should stop your son doing homework in the evening and let him have the good rest for the eyes.
Short time of reading under candlelight can not cause damages of vision, but long time of reading under low light can cause a lot of eye problems, especially for children. It can be easily get myopia for children reading under candlelight. Try your best do not let your child reading under candlelight for a long time.