Bladder pain and urinary straining- does he have any other symptoms?
Please give more details, such as:
your son's age Antibiotic use in the past 1-2 years
Other medications or other conditions he has had doctor treatment for
typical diet over the week
skin condition and if he is supplementing vitamin D (if so, how much, if not, get a hydroxy-vitamin-D blood test done!!)
Ask if he is sore along the back sometimes as well, especially up under the shoulder blades (if so, if it is most often on the right or left sides). (these are hints to his kidney health)
Bladder and/or kidney pain may indicate there are small oxylate stones that need to exit the body. A stronger magnesium and B1 intake is an excellent remedy against this issue, and magnesium is one of the major electrolytes that stop calcium stones from accreting in the kidney tissue.
Adequate vitamin D3 levels are absolutely necessary to absorb magnesium -and- all essential minerals in the digestive tract.
1-2 organic lemons' worth of juice in water each day (up potassium and magnesium intake, along with natural vitamin C. Give him a nibble of the rind so he gets the complete vitamin C complex, which comes in both the juice and the rind/white meat ).
Vitamin D3 2000-3000 iu/day with a whole-food multivitamin.