Why does my puppy run off with socks and shoe?

I have a 5 month old Lhasa apso, he is forever running off with socks shoes knickers and now chewing the doormat he takes them in the garden and starts chewing them, i have tried saying no in a strict voice and tapped him a couple of times he gets walked regularly and lots of attention, he has lost a couple of baby teeth thought the chewing could be maybe his gums being sore.....any advice please would be really grateful ty for listening