Please - need help with getting autistic son dressed

Hi - please, I am looking for advice for my 11 year old son & others with experience with similar issues

My son has been formally diagnosed with autism, lower IQ (67) and ADD. Not sure if there is anything else we may be missing. He has been seen by 2+ psychologists (including on a regular basis), occupational therapy, math and reading tutors, pediatricians etc.

I am his dad - 38 yr old with bipolar 2 and likely some more mild depression and anxiety; on lamictal, citalopram. My maternal grandpa was diagnosed with bipolar 1.

My son is getting more challenging - not easier. 80% of his time awake he is a good kind boy.

The biggest problem we have is he absolutely 100% locks up with an inability to perform maybe 70% of days while getting dressed in the morning. We have tried to-do routine checklists, traditional punishments (taking away pokemon cards, etc), time outs, some mild spanking, etc. 

This is taking a significant toll on our family and truthfully; breaking our hearts and driving a wedge in our lives, etc. This is also creating an inability for us to get him to tutors, school, church, etc. There is definitely some correlation between him not wanting to do an activity and this problem (for example; if he needs to get ready to go play, do something fun, go to grandma's etc; there is usually not a problem - but if it is for school, church then more likely that there is a problem). I am really hoping not to have to take him out of our home and place him in some type of a treatment live in center.

It is interesting because sometime he will have been dressed for hours, and when it is nearing time to go - everything becomes a problem. I am sure he has some clothing sensory issues - this is normally part of the problem (everything "feels weird" - pants too tight even though they are 2 sizes too big, shirts or collars too tight). He generally has developed a few favorites (even though for example with underwear a pack comes with 6 pairs - he will only wear one or 2) - he also does better with smooth clothing (Under Armor shirts, exercise pants - will not touch jeans). We have tried some specialty tight seamless autism shirts - really I don't notice a major difference with those.

He becomes wildly frantic and emotionally/mentally paralyzed; unable to take any action.He will sit there (I assume in an autistic meltdown); saying "I just don't know what to wear"; "I need help"; "Will you help me", etc. We attempt to help and 100% of the time everything gets refused - "I don't want to wear that" and so on after cycling through every piece of clothing (many times with annoyance, irritation, "you're dumb" for proposing it attitude; sometimes does things to escalate the issue to demonstrate his discontent with us; sometimes very mildly violent with us). If we try to force clothing on him; the panic increases; physical resistance - and immediately the clothing comes off. 

Sometimes the only way out of the house is to grab some clothing and literally carry him out of the house and let him dress in the car - of course with sheer emotional panic and anger on his part.

I feel threatening with losing a toy, etc only really makes it worse; but me and my wife are only human and don't know what to do even after many hours of psychologist visits and such. For example today; after missing his reading tutor appointment and 30+ minutes of meltdown; I set a timer for 4 minutes; and told him to get dressed within the timer or he would lose his opportunity to go see the new Pokemon movie. Nothing happened (meltdown, "I don't know what to wear", etc). I finally had to leave to work (mom will take him to school) - he was in sheer panic - chasing me down our apartment stairs in his underwear - "don't take away the pokemon movie" - just waiting for the police to be called on us. Always looking back - I feel like the threats make it worse; but we don't know what to do. I feel he must have some significant anxiety issues; but they are not formally treated. 100% of the time; he sees himself as the victim, sees no fault in himself - we cause the issue; we are mean; bad parents, etc.

Please help or share any advice possible. Would love to know if you or someone you know has this problem and how to help.

Thanks