If it returns to normal in 2 hours without having any issues then it can be normal. Ask if he is taking any performance enhancing drugs like sildenafil etc. Prolonged erection can be side effect of these drugs. Also if he is able to perform sex in a way both of you enjoys then its ok, nothing to be worried of.
Next Steps
Consult an urologist urgently if erection persists for more than 4 hours.