Firstly, I would get him checked over at the vets, just to be on the safe side.
It could be a number of things:
- Dogs can be funny and make weird associations eg something hurt or frightened him at the same time the baby started to cry and now he has made an strong association between the two, so is stressed and worried every time baby cries.
- He is naturally a sensitive dog and is taking a long time to adjust to his new family member.
- He could have an ear infection so the crying actually physically hurts him. One of our three dogs has a recurrent ear problem and sometimes howls if one of my 2 boys (aged 6 and 4) cries, its always a sure sign her ears are bad and she needs treatment.
One thing to remember is not to fuss him if he is looking or behaving at all stressed or fearful as this will just reward the wrong behaviour. Lots of praise and attention when he is relaxed and settled in the presence of the baby is exactly what you need though.
Maybe try sitting quietly with him in the same room as the baby, with baby asleep at first if he won't come in with the baby awake.
As long as he is sitting/lying quietly try intermittently dropping small (thumnail sized) treats on the floor. This will hopefully make him reframe being in the vicinity of the baby as a positive thing and will move his focus away from the baby and on to you.
You will need to work up to having him in there when the baby is awake/making noise. Don't force this though, wait until you can get him to stay calm, focus on you and respond to simple commands with baby asleep, then move onto happy awake baby, then again, when he is used to that and responding to commands in that situation you can up the ante to while baby is in noisy mode.
I would definitely recommend the DAP diffusers, but probably a collar rather than a plug in, as this will help calm him wherever he is rather than in just one zone. I have just bought one for my collie cross (firework phobia) along with some valerian compound drops from Vet UK. They were the cheapest I found online. I have used this combination a few times now and it definitely helps our dog.
If you really feel it is stress/fear related, a combination of the DAP collar and valerian might be just what you need to calm him down and reframe his association through praise and reward.
Vet UK