The reason why most bonding companies will not deal with these types of cases is that even once the county bond is paid, ICE will then be notified to come and pick up the detainee. In other words, even if you or a bonding company pays for the county bond, your boyfriend will not be released into the public. Instead, he will continue to be detained until ICE comes to pick him up in which they have 48 hours to do so from the time it has been notified. During this time and if your boyfriend is not subject to mandatory detention, ICE will then set its own bond. If you feel ICE's bond is too high, you can hire an immigration attorney to request a bond hearing with the immigration court that has jurisdiction and request that the bond be lowered or if he could be released on his own recognizance.
If your boyfriend is undocumented, that may be the problem. It is difficult to find a bonding company that will post bonds on criminal cases for a person who is undocumented. It makes more sense to wait until the criminal case is finished. Then, you can try to get an immigration bond. You have to make sure that you have a criminal attorney who is aware of the immigration consequences of crimes. But every situation is unique. You need to speak to the criminal defense attorney.