Thanks for the question,
While I cannot diagnose this via the internet I can tell you the following :
1. In my experience your dog's lymph nodes in his neck being swollen would be very unlikely to be related to mould in any way.
While your dog's lymphatic glands can become enlarged for a variety of reasons such as local infections you would always in these circumstances be on your guard about a disease called lymphoma which is a very serious form of cancer which is quite common in the dog and the first sign would be enlarged lymph nodes under the neck. Such dogs commonly pant and wheeze as well. While I do not want to be melodramatic this is not a disease you would want to miss so you should see you vet over the next couple of days if only to rule lymphoma out.
Since I have mentioned lymphoma I know you will be wanting to know more so I will tell you what I know about it. Hopefully your dog has something much less serious of course ...
1. This is a disease which will present with swollen lymph nodes in the neck and elsewhere. The only sure way to diagnose lymphoma is via a lymph node biopsy, you can suspect it on clinical signs but you will not be 100% sure. Blood tests are an aid to diagnosis but often do not present the whole picture.
2. There are a number of ways of treating lymphoma, one of which is by the use of a drug called prednisolone alone, this is given orally. If you go down this road your dog is likely to have a six week to two month remission period, during this remission period these dogs will be close on normal and have a good quality of life. At the end of the remission period the disease will come back with a vengeance and normally such dogs will die very shortly after that.
3. There are more advanced treatment plans involving standard chemotherapy drugs which will give significantly longer periods of remission. However using steroids alone previously will very much reduce the effectiveness of them if you decide to change treatments. So you have to decide which way to go from the start.
4. On the chemotherapy protocols I have undertaken the side effects I have seen have normally been quite minimal. What you would normally achieve would be six to nine months of remission and normal life but then the drugs become ineffective and the dog slips away. Cures are in theory possible but I personally have rarely seen any but you would have a much better chance of success if your dog is dealt with by a specialist oncologist.
Here is a link to a very good web site which covers the subject in depth so you can research it all a bit more : LINK
If I have not covered your question fully enough or you would like to ask more I will be online for the next hour or so and I will be at your disposal
Scott Nimmo BVMS MRCVS