Hi, bubbles in urine is common in rapid urine urination and in mild dehydration. But proteinuria ,UTI, fistula can also cause foamy urine. If he infrequently notice bubbles in his urine then it is problem not something he need to be overly concerned about. However, if his urine frequently bubbles or has a frequent foamy consistency he may need to contact physician especially if these symptoms are combined with additional symptoms of kidney disease . Since he is diabetic I suggest you to take him to urologist.