Babygates!
I've not had experience of introducing a pup / another dog and them not getting on. (I know other COL members have thou
) , I'd suggest keeping the two apart most of the time, using baby gates etc. How long are you 'puppysitting' for?
Have the dogs met before?
(I'd imagine having not met the dogs, that the pup is just trying to instigate 'play time' with your dog rather than the issue being 'hormonal' as the pup is so young.)
I hope you find a workable solution for you soon