My son (15 yrs old) had spinal fusion and costoplasty 14 days ago - he is still quite weak and is getting headaches - is this normal?

It can be: The size of the surgery and the total blood loss can have this effect post op. The headaches could be pain or medication related or even muscular on origin if the incision went up to his neck. Sometimes headaches can be related to a spinal fluid leakage that is a headache worse with upright position and better supine. You can check with your surgeon. Nutritional status can play a role too.

Yes and no: Children usually bounce back quickly from this type of surgery. If he's not making improvements over the next couple weeks you should definitely let the spinal surgeon know what is going on.

See your surgeon: Children and adolescents tend to heal very quickly. While this may be normal i agree that you should consult your surgeon. You need to be certain no post-surgical problems are involved. Weakness and headaches may be something and simple as diet or viral, however he should be evaluated.