What has been gifted is not equity shares but Bonds. The income from thes bonds is now to be assessed in the hands of the holder itself as he is no longer a minor but an adult now. A transfer from a father by gift to a MINOR son, even though a transfer technically will not attract any income tax on the transfer as father is the natural guardian of minor and it is as good as transferring the bonds to himself. The interest or income is clubbed during minority of the son.
Now son can sell bond without father's consent as he is major and the asset is his own to appropriate. The income is also his. When he sells it you only have to count the date the father purchased (not transferred) the bonds for purpose of long term or short term asset. If this period is over 36 months you have to take long term capital gain without indexation (difference in selling and purchase price) and pay tax @ 20%. If less than 36 months it is included in the income as short term gain and taxed. This is usually treatment for Bonds. Maybe there could be a special treatment specifically for sbi flexibonds as far as special rate of taxation is concerned. Refer to the offer document.