sounds like you got some great advise so far. i would get the lowest mini bars you can get and by all means cut them a little off each end… to make sure his hands are gripping on the part that is equal with his shoulder width. they shouldnt be out too far that would make him unstable as far as balance goes. my husb always trimmed the bars a little bit..dont worry you will always get your moneys worth out of them when you sell them. they are custom and there is always someone who has a little guy who is looking for them.
agree to turn the stem around facing the seat to shorten the tt length.
a jr frame seems a bit large for his size but with the modifications it should be fine. the cranks seem a bit long. if his hips are moving up and down(choppy) when he pedals they are prob too long.i would try 145-150’s.
building a bike for a little guy is an education in itself. unless you have done it you have no idea…. its confusing at best but always ask there are plenty of dads/moms who have been there done that.
remember in bmx never buy a bike that your rider will “grow into” it should always fit him for this season. what we used to do is get the frame/ equip handed down from someone we trusted who took care of their bikes… then you sell your outgrown items and buy the next size up.. always consult other parents but see what feels comfortable on your son, after all he is the one who is riding.
hope this helps- my son started when he was 4 1/2 on a 16″ heavy hand me down and we got lots of help building his first “race bike” that didnt have to break the bank either… remember he is 5…not going to the olympics…