There isn't sufficient information to suggest a specific diagnosis such as if your boyfriend had any trauma to the foot prior to o was it spontaneous development over the foot. I would advise you to visit the ER or orthopedician so that the foot can be examined and undergo investigations such as X-Ray, CBC, etc. to find the underlying cause and have it treated accordingly.
Next Steps
Follow the steps as advised
Health Tips
If condition persists or worsens visit the ER at the earliest