Sandhyavandanam is the identity of a brahmin. There is a lot of power in it. It increases the mental strength because of the pranayamam. It increases the divinity in us because of the gayatri mantrams. It makes us humble as we do prayashittam daily and dedicate our actions to the Lord.
Kaayena Vaacha Manaseindriyairvaa
Buddhy Aatmanaa Vaa Prakrteh Svabhaavaat
Karomi Yad-Yat-Sakalam Parasmai
Narayanayeti Samarpayaami
The following is the meaning of the kayena vaacha
Whatever I do with the Body, Speech, Mind or the Sense Organs, Either by discrimination of the Intellect, or by the deeper feelings of the Heart, or by the existing Tendencies of the Mind, I Do them All (i.e. Whatever work is to be done) without Ownership, And I Surrender them to the Supreme Guru.
The Pranayamam starts with Om Bhooh Om Bhuvaha etc and after one full gayatri mantram , we chant Om apaha, jyothi rasa, amrutam brahma, bhurbhuva suvarom
The whole thing should take thirty seconds and 10 pranayamams should take minimum 5 minutes. There is no meaning of chanting pranayamam in less than 30 seconds. It is a breathing exercise in disguise.
Like wise, gayatri japam should be chanted slowly. One gayatri mantram should take around 10 seconds.
All these will take around 20 to 25 minutes for a full Sandhyavandanam. If your son is overburdened by the homeworks and other academic schedules, I suggest that he does only once in a week , ( say ) Sunday morning. You should also do the sandhya along with him. This will increase his interest in the ritual. But it has to be done for 25 minutes elaborately. God bless him. All the best.