Is it permissible for him to buy a computer for his son using interest he has received or keep it in return for waiving debts owed to him?

Praise be to Allah.

Firstly: 

It is haraam to deal with riba in all forms and this is a major sin. Allaah says (interpretation of the meaning):

“O you who believe! Fear Allaah and give up what remains (due to you) from Ribaa (from now onward) if you are (really) believers.

279. And if you do not do it, then take a notice of war from Allaah and His Messenger but if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums)”

[al-Baqarah 2:278-279]

It is essential to repent from it by giving up the sin, regretting what has happened in the past, resolving not to do it again and giving up the haraam money by spending it on charitable causes and in the public interest. The penitent does not have the right to benefit from it by using it in his own interests, such as buying food or drink or accommodation or spending it on his family or education fees, or on warding off harm or injustice from himself, such as paying the costs of compulsory insurance or any other kinds of taxes and levies. 

Based on that, you do not have the right to spend from this riba on obligatory maintenance of your son, such as his food and drink or the costs of his education. With regard to buying the equipment that he is unable to buy and which is not included in the obligatory maintenance, it seems that this is permissible, and it is also permissible to pay off his debts, because the father is not obliged to pay off the debts of his son. 

Secondly: 

It is permissible to give this interest to a poor brother if his maintenance is not obligatory upon you. See the discussion of that in the answer to question number 81952. 

Thirdly: 

It is not permissible for you to take this interest in return for waiving debts that are owed to you, because there are benefits for you in that and it is in your interests, because debts may be paid off or the debtor may not be able to pay it. It has been stated above that the penitent is not entitled to benefit from the interest. But you can give it to these debtors, and if they use it to pay off the debts that they owe to you, there is nothing wrong with your accepting it. 

We ask Allaah to forgive your sins and to accept your repentance. 

And Allaah knows best.