Read the history and understood the problem your boyfriend has. Dysuria
or trouble or pain at urinating after sex can be due to few problems:
-If your boyfriend has urethritis
of any sort, the act of sex stimulates the urethritis part so much that there will be pain if he tries to pass urine after sex.
-If you are using any spermicidal jelly in the vagina, your boyfriend can have the burning urination.
I would suggest you both the following:
Get a course of an antibiotic and metronidazole
for 5 days, no sex for 10 days and to use condom for about three weeks.
If there is less response then both of you need to undergo internal examination at the Gynecologist and your boyfriend at the Urologist
to have physical examination, investigations and proper treatment.
I hope this will solve the problem.
In the meantime ask him to increase his water intake.