it depends. my son is also 12 yrs now and has played for >5 years. during last 4 yrs trained at OAC, he started with 26" junior one for only 6 months and switched to the regular adult racquet under coach's supervision, when he was 9 yrs old.
What I said is for a pure beginner to start to learn how to play tennis from square- one as your son did (7 years old?). After 6 months, and if the kid is strong enough, he/she could switch to the regular racquet and balls. However, many kids keep play progressive tennis until 13 years old. There are progressive tennis tournaments in the Hull side each year. I saw that some 5-6 years old boys and girls playing, very impressive.