Thanks for sharing your query with us.
Darken circle area you have mention under the tooth 35 can be because of bone loss under tooth which occurs because of any Periapical ( below the roots) infection;here which relates with the SWELLING or LUMP on face in that region.
It can be any dental cyst
or an abcces. But can only be conformed by viewing the x-ray only so i can not tell whether it is cyst or abccess or any other pathology.
Also you said your son is 9yr old so in 35 region,the developing posterior tooth in the jaw also shows round encircled tooth below the upper tooth which is seen in the oral cavity,the roots of which are seen short,resorbed and uneven in the x-ray. But the teething
process do not accompanied with lump unless there is infection to the tooth.
so I would advice you to send the x-ray of your son so that we could find the problem.
Hope you satisfied.