Hi thanks for your question
Your son had rectal bleeding
four month back, now he is complaining of incomplete evocation along with swelling in abdomen.
Most probably your son bled from a pile mass or anal fissure
and he has incomplete evocation due to painful defecation or due to constipation
The lump he is feeling in his abdomen is most probably retained fecal matter in rectum.
Get your child examined by a physician, he needs through physical examination along with investigation, espicially photocopy and or sigmoidoscopy
(examination anal canal
and large intestine with small cope).
Hope this answers your question