Question: my brother (13 yrs) doesnot speak properly (does not speak sentences insted speak only words).His iq was 78 at the age of 5.Is he will be able to speak normally.what should we do???

his milestones were some what late like he started walking at age of 1.9 years, start speaking few words at the age of 3.But his fine motor milestones was good like he can pic small ants,pins etc at early age.His general understanding was also ok like he understands things quite good.but he dont want to do works which requiers mental work. His hearing power is very good and does the thing said to him .He can do simple calculations and mug up the tables. He is able to write things like upto 2nd std,like remember spellings etc.But now he is grown enough and dont want to study we dont know whether is he is doing this intentionally or really he is unable to study.He can play cricket,badminton etc .He is able to handle mobile like calling to their all family members by seeing their name in contact,plays simple game on mobile,operate TV,DVD player etc.
He is very normal at home but infront of strangers he behave very overexcited,like he sometimes hit childrens ,poke them and is unable to handle at that time.
When we try to make him speak he tries to skip that area.what should we do??(he is on treatment and take medicine BRAIN WISE and Cogenium)
can he will be able to live a normal life like getting married and having children??