As you mentioned you want to share Internet from Mobile to your tablet using your phone's Bluetooth tethering.
So first lets understand a little bit about Bluetooth services: A device which need to access internet via Bluetooth from other device should have Bluetooth PAN Service (BT Personal Area Networking) installed/enabled. While the device offering the internet should have the Bluetooth DUN Service (BT Dial-up-networking Service) installed/enabled.
How Connection is established:
Device with (with BT PAN) establish a wireless dialup connection by dialing a certain code (#99 for GSM devices & #777 for CDMA devices) required to make use of the modem of the internet providing device (equipped with BT DUN). If all goes well connection is established.
Since most of the mobile phones and tablets are only equipped with BT-DUN Service and not with BT-PAN Service.. Your tablet is not able to establish the internet connection using your phone's mobile data service. But they can however do other BT exchanges using BT FTP (file Transfer) BT Audio (for headphone/voice) etc.
Since most of the PC's and Laptop does have this BT PAN service enabled, they are able to use the BT tethering from devices offering it.
Solution for You:
The most hassle free and easiest solution for you is to use Mobile Phone's WiFi Hotspot for sharing internet to your tablet. this should work well, provided u arent using a illegally unlocked phone (as hotspot functionality gets disabled in some unlock cases like this)
Hope that answer your query. If you still require any help feel free to let us know, we are always here to help :)