Pups don't have full bladder control till the age of 12 weeks, that is there is no message from bladder to brain that they need to go. He has only just begun learning in effect aye!
It can take up till 6 - 10 months of age to fully house train a pup. You have obviously been able to let your pup outside in good weather but now you are inside you are realizing he is not house trained at all.
Easiest way is to confine to a small area when you are not able to supervise constantly. Place your pad within that area, put it where your pup consistently chooses to eliminate and chances are he will go on the pad within 12 hours.
Now you go into Serious house training, a pup needs to go after....
A 12 wo pup can hold on for apx 3 - 4 hours, so you will need to take him out after each of the above as well as every 3 - 4 hours.
Be consistant, take to the same spot outside every time and give praise when he eliminates in the right spot. Never punish as they have no cognitive recognition, that is they don't understand reason, only positive reward.
Good luck and keep being positive, good house training in the first 6 months makes it much easier for you and your adult dog.