I would want to know more about the training level of the older dog.
Your boy is about just starting out in adolescence - he may sail through with no issues, but he may need a lot of training input from you over the next few months. If the older dog is well trained and good with other dogs, he could well be a positive influence on your boy... but if he has training needs of his own or isn't that well socialised you could end up with twice (if not more) the work.
There is no guarantee that the two dogs will get on - even if they tolerate each other you may need to keep them separate when unsupervised for quite some time, so don't go into this with an idealistic picture of them both curling up together when you leave them. Obviously they could be the best of friends and everything could be fine, but there's no real way of knowing that until they have been living together for a while.
I think, assuming the are no major training needs for the older dog, and he isn't going to be bringing bad habits for your boy to pick up on, you do have the time to devote to two dogs (especially important in the settling in period) - and adding an older dog is certainly a better proposition than adding a puppy considering the age of your existing dog - but it may not be as plain sailing as you are hoping, so you need to be prepared for this if you do decide to take the older dog on.
Is there any way you could trial a visit of the other dog, to see if they are likely to get on (though bear in mind some issues can take weeks/months to become apparent) - a trial overnight visit might at least answer whether the two of them can get on within the same house.
My main concern would be the level of training of the older dog - if the owners are having to give him up due to lack of time for him, he may well not be that well trained, and may have bad habits which have arisen through boredom... I think this is what I would want to know much more about before committing to the dog.
Sorry if my post sounds negative - and I hope none of my fears are justified at all - but I do think if you go into this you need to do so knowing all the potential issues so that you can be prepared for any problems.
Good luck with your decision.