Brewer's yeast works well on repelling fleas. Also garlic in moderation. Too much garlic can be toxic. A spray of lemon juice and water mix can also help, being careful not to get any in the dogs eyes.
Fleas are a symptom of interrupted health. Use lemon juice (1 tbs) in a pitcher of water as a bath,or 10ml of almond oil with about 5 drops of lavender.