As the owner of a 4 yr old cocker and Mum to a 14.5 month old, I know exactly where you are at.... and I don't find the suggestions of a toy dog, or comments on his reaction to the toy dog, particularly relevent... at 14 months old your son has no concept of empathy... he cannot understand that his actions are worrying your dog, and certainly won't equate how he treats a stuffed toy (in our house Winnie the Pooh is alternately hugged and thrown) to how he should or would treat a real animal - most things are thrown at this age...
We have been "managing" all interactions between Isobel and our dog (Honey) from birth... Isobel loves to sit with a bag of treats and feed Honey, but knows to ignore her unless we are managing the interaction... this is something you really need to instill in your son... do it gently and consistently... when you are playing with him, make a point of saying "lets come over here so the doggie has some space" or "we don't walk up to the doggie, let's go this way instead". We use playpens, gates and room dividers to ensure dog and baby are not left unsupervised with access to each other. We encourage gently stroking when we know Honey will be receptive to it.
To be honest, smacking your child's arm to stop him grabbing the dog's fur is only going to encourage him to mimic smacking behaviour.... if your son does grab at the dog's fur, hold his forearm above the wrist - you can prevent him pulling back and hurting your dog, and this action naturally encourages a child to loosen their grip... you can then help your son to stroke the dog, saying something like "gently with the doggie". Then distract your son away from the dog, and praise your dog for tolerating your son's attentions...
You might find this a useful article to read -
http://dogsandbabies.wordpress.com/2011/02/24/helping-toddlers-not-be-magnetized-to-dogs-part-3/Just another little comment - please be very observant of your dog's body language around your son, and step in at the slightest sign of discomfort... you don't want to create a situation where your dog feels he has to defend himself against your son because he isn't confident that you will keep him safe. Honey will often growl/grumble if Isobel is approaching her... it's not a worrying growl, as her body language remains utterly relaxed, it's her way of saying to us "Help" - depending on the situation we either just supervise more closely, show Honey a way of getting herself out of the situation or we distract Isobel...
We do regular stints of Isobel approaching Honey with a treat, so that Honey learns that Isobel being close by is a good thing...
But you absolutely must ensure they have their own space and learn to ignore each other before you can ever hope for them to be friends...
