The majority of kidney stones are made of calcium oxalate, resulting in about 85% of all diagnosis.
If this is this case with your husband then he should try a kidney stone diet thats low in oxalate foods. Compare what he is eating to the list below.
Having said that that major cause of kidney stones is dehydration. If he is working out doors a lot especially in the summer and not staying hydrated then this is the most probable cause. He should be drinking at least 2 liters of water a day. His urine should remain almost colorless or a very pale yellow. The darker the color the more dehydrated he is.