i don't think using a leash out of elastic material is a good idea.
if you dog can pull the leash to get more length and go after another dog, you have less control over him and if he comes the end of the extra length the danger of being pulled off your feet by him is still there.
Plus the dog shouldn't even get used to tension on the leash in my opinion.
personally I wouldn't let a leashed dog play with an unleashed dog, because they can get tangled in the leash and hurt themself or they can't avoid the unleashed dog as well as if they were without a leash.
I think it is much easier accept for them that there's no contact with other dogs when on-leash than when it's sometimes allowed and sometimes not.
I'd try to find an area where he can play safely with other dogs (preferable adult dogs of which you know they've got decent social behaviour) witout the leash.
if you dog can pull the leash to get more length and go after another dog, you have less control over him and if he comes the end of the extra length the danger of being pulled off your feet by him is still there.
Plus the dog shouldn't even get used to tension on the leash in my opinion.
personally I wouldn't let a leashed dog play with an unleashed dog, because they can get tangled in the leash and hurt themself or they can't avoid the unleashed dog as well as if they were without a leash.
I think it is much easier accept for them that there's no contact with other dogs when on-leash than when it's sometimes allowed and sometimes not.
I'd try to find an area where he can play safely with other dogs (preferable adult dogs of which you know they've got decent social behaviour) witout the leash.