Thanks for your query.
What do you mean by lump in base of skull ? I ask because base of skull is not visible from outside.So I may assume that you want to say the lump is in the nape of neck or under jaw.
How is the swelling? Hard or soft? Is it mobile or fixed to underlying tissue? What is the size, colour ?
It may be a lymph
node , a neoplastic growth , a cystic swelling or anything else.
You need to get your son a checkup from your doctor.USG, biopsy
or FNAC may be needed as your doctor asks.
Regarding the black out episodes , there might be many reasons.Yes, low blood pressure or dehydration
may be the cause. There are other causes as well . A check up from a neurologist
is needed . CT scan
of brain may be needed.
Hope this helps.
Update here about the condition of your son.