Trouser legs, tiredness and a visiting pup

[Toadpig]

I have read alot about tired pups getting overstimulated and about putting them in their crates when this happens. However my issue is with a pup who visits for two or three hours twice a week. My sister has an 8 week rottie pup, her first though not her first dog, and they both visit regularly. After a couple of hours of quite good natured visiting the pup tends to attack my trouser legs when I walk. I have successfully managed to redirect his biting of hands, arms, etc with a toy (as suggested on this forum) but he will not let go of my trousers. He starts to tug and growl and will not let go even when I try to give him a toy instead. A firm 'no' makes him growl more and if I try to remove him by lifting him away from the leg he gets nasty and starts to bite my hands. I think the dog gets this way when he gets tired but I do not have a crate and my sister cannot bring hers every time she visits. Does anyone have any suggestions for dealing with this? I have no experience with rotties and am really in the dark. Thanks in advance.