Is pain in the same side with which he is planning bedminton, if yes than we need to correct his playing techniques to avoid this type of problems in future also need to have proper diagnosis first

Hello He is a badminton player so there may be acute injury or may be some muscle sprain while playing and he kept playing so that causes constant pain as you mentioned. Where is he having pain in right forearm or left forearm? And does he play with his dominent hand? Ice will definitely help him if it is immediate pain but now pain is chronic so ice doesn't help him much. Does he having muscular pain?? Or is it on joint?Please mention exact pain site. Myofiscial release is a technique which helps him a lot Secondly Tapping is another type of treatment which helps him.

Hi, can you give more details. Swelling?? Pain to touch?? Pain with movement?

Since it is a growing age and the pain has been there for nine months, he should consult a Physiotherapist or Orthopaedist or Sports medicine physician for complete assessment and treatment.

Correct his techniques, proper warm up n cool down. Avoid repitive use of the hand, give enough rest. Ice it n stretch it

Do shoulder extension stretch. It may help you

Need to ask more questions like have you taken any treatment for this, is there any history of fall or injury in past, is the same pain bother him previously, since when he is playing the badminton in that time is anything he noticed.. Need to check what is the cause of pain. For better treatment results need proper evaluation and diagnosis of problem.it is better to take him to nearest physiotherapy clinic for proper assessment and treatment. Apply ice pack three times a day 10 min-15min everyday. Use pain relieving gel like volini gel or diclofenac gel.