Misha did this as a pup shortly after I got her (and when she was first allowed out after jabs). A friend was puppy sitting for me and decided to take her dogs and Misha to the local common (I hadn't even done that yet) and her dogs were much bigger than Misha and whilst they didn't mean any harm I suspect that the play was quite rough and may have scared her.
The next day when I took her for a walk she screamed her head off when she saw a black and tan cavi, something she'd never done before. Poor dog hadn't done a thing and had just stood there. This happened a few more times when we met dogs and she was quite obviously scared of other dogs, especially black dogs. After that I booked her in to a local doggy day care where she could play with puppies the same size as her. We only did this for 1 morning a week for a few weeks, and after that she was fine - still a little wary of other dogs initially. Roly has really helped her on that front as he is a real diplomat when it comes to meeting other dogs. He would always go and meet dogs first and then Misha would follow. Nowadays she is fine meeting other dogs, but still a little stand offish when meeting black dogs (by the way my friend's dogs were both black).
Only thing I can suggest is to see if there are any socialisation groups for dogs with gentle dogs of a similar size that you can go to that would help Luis build his confidence. Sometimes vets have puppy groups, and while Luis might be a bit old for them it wouldn't do any harm to go and visit where the dogs can play in a controlled environment. Alternatively, if you know of a confident and gentle dog that Luis is comfortable with you could possibly join them on a walk every now and again to help his confidence with other dogs.
Given his age I suspect that it is the 2nd stage fear.
