Hi..
Welcome to HEALTHCARE MAGIC..
As per your complain
sinus infection or Sinusitis along with sore throat tha can be due to
post nasal drip and also headache and cough are due to Sinus infection only...
Taking multivitamin alone with not resolve the problem and he has to take proper medical care with a course of antibiotics ..
I would suggest you to consult an Otolaryngologist and get evaluated and a thorough clinical evaluation
and investigations like PNS view x ray or MRI scan can help in diagnosis and treatment can be done
accordingly..
You can be advised a course of antibiotics, anti-inflammatory painkiller like Ibuprofen, decongestants
like Pseudoephadrine, do saline
nasal irrigation, steam inhalation and use a humidifier in your room..
You can also be advised decongestant nasal sprays containing
Xylometazoline or
Oxymetazoline..
Avoid spicy and chilled foods..
Do warm compresses around nostrils..
Hope this helps..
Regards..