A home invasion is a very serious charge. Your son needs a very good criminal defense attorney! If he/you cannot afford to hire your own attorney, one will be appointed for him. I assume a qualified public defender or attorney on an court approved panel will be appointed to represent your son.
If you are looking at hiring a qualified criminal defense attorney you might try the local Bar Association. Most counties have referral panels administered by a local Bar Association; if not, they may know to whom to refer you. Many of these panels have attorneys who agree to represent borderline indigent defendants on agreed upon rates. Perhaps you can find such a panel or attorney.
You do not want someone who is not qualified even if that is "pro bono." If someone volunteers to represent him, check him/her out to make sure he/she is qualified - you need to know how many trials in serious cases such this the attorney has conducted, at the very least. Do not short-change your son's defense in this instance! There is too much at stake!
I would echo the comments above. The criminal system has in place procedures for representation to be provided to those who are unable to afford private counsel. If your son financially qualifies, he will be appointed an attorney at no cost.
You do not want a budget criminal defense attorney - you'll get what you pay for. Criminal defense attorneys that take on cases for low fees operate on volume and cannot devote the time necessary to any particular case, making you in no better position than with a public defender AND you've spent your money unnecessarily. As professionals, we have our time, knowledge and experience to sell. An attorney that works the time necessary for the defense of your son at a free/reduced rate will quickly go out of business.
The other problem with hiring a cheap attorney is that if you pay all you can for them, it leaves no money for potential investigation. Some cases will require additional defense investigation. If the attorney has to reach into their own pocket or take from their fees, there's the risk they will short-change the investigation. Public defenders' offices have investigators on staff and will do the work the attorney deems necessary.
Your son has the choice of accepting the services of the public defender and/or appointed counsel, hiring a private attorney of his choosing if he can so afford or... he can represent himself.