What part of bipolar are you trying to control? If you are trying to treat psychosis then you will have to stick to an antipsychotic. If you are trying to keep his mood stable then lamictal is a good choice.
From what I've read - always best to do your own research though. Clozaril is considered one of the best antipsychotics out there, not easily replaced unfortunately. However dangerously low white blood cell counts can not be ignored. Without your white blood cells you can get infections and your immune system is shot. And he is on Abilify which is another antipsycotic but considered a weaker one in potency generally. Geodone (Zeldox in Canada), and Ziprexa are considered stronger, but Ziprexa packs on the pounds - really packs on the pounds and they all can cause diabetes with long term use.
As Iladvocate said though they have some drugs in development which will hopefully come to market soon. Maybe they will help your brother.
I will also add that right now they are developint new treatment modalities that are not yet FDA approved and in the early stages of testing. One of the more promising ones are the NMDA receptor modulates (google "A New Class of Antipsychotics, Psychiatric Times") some of which are in Phase 2 FDA study. I am in one of the clinical studies. Its not known whether they will complete the studies or not but it is known over time more helpful and safer treatments will be developed so its worthwhile to keep yourself updated for the future as well.
Yes the use of antipsychotics is clinically complex in people with developmental disabilities but they are sometimes needed to treat a secondary psychiatric disability which is what you are describing. Clozaril is not used as often (although it is highly effective) as it would be because of the potential of blood dyscreias. A person is generally considered a candidate for Clozaril when no other antipsychotic is able to treat their psychosis, they cannot tolerate all known antipsychotics or they have the long term movement disorder tardive dyskinesia (in which case Clozaril is the only antipsychotic that wont worsen it).
A psychiatrist generally doesnt transfer a person off Clozaril unless they develop blood discreas. If they do they might use antipsychotics that have been FDA approved since the person was started on Clozaril. It should be noted each person responds differently to each medication. This is the information I was told as a person who was on Clozaril.. All this should be discussed with a psychiatrist as they are the only one who would understand the clinical specfics of this and know how to follow up within their professional discretion.