Hi Tracey,
There are basically three causes of swollen lymph nodes...
1. Infection (the most common in children)
2. Malignancy (the least common in children)
3. Autoimmune disorders
I remember having swollen lymph nodes as a child (as a result of infection), especially in my neck... it was uncomfortable to touch them. You can usually tell if infection is the cause of swollen lymph nodes by taking a round of antibiotics, but I would avoid that as long as your son is not too uncomfortable. IMO, it's not worth messing around with the good flora in his intestines.
In RARE cases, kids can develop lymphoma (or another advanced malignancy) or a systemic autoimmune disorder like lupus. Is there any history of autoimmune disorders in your family? AARDA.org has a good list if you're not sure. Keep in mind that lupus is much more common in females than males... it also tends to be accompanied by symptoms like joint pain, kidney damage, rashes, headaches, etc... Lupus in a 12-month-old would be extremely rare. Same for lymphoma (most lymphoma patients are age 50 or older).
As someone with a "rare" disorder... who also lost a child at a "rare" time in pregnancy (17 weeks)... the word "rare" is not as comforting to me as it might be to a "normal" person
I do think the lumps are something to keep an eye on for the next few weeks, but try not to panic. I'd say the chances are about 99% that it's an infection.
Heather