Wow. That's alot of stimulates to throw at a kid. My daughter is 3 1/2 and going through the same thing. It got so bad that she once tried on and off for 8h with little result. She cried that it was too painful. She hadn't gone in 3 - 4 days. Our pediatrician put her on Miralax 2x a day. We mix it in with her dinner time drink. Make sure the liquid to mix ratio is per directions. Hubby made the first glass, was getting ready to hand if over. I didn't think it was enough liquid. We measured it, and sure enough it was half the amount of liquid that the directions said to use.
I don't think I would do the Milk of Magnesium too. For me that would be too much stimulant, esp. if its within the timeframe of having given the Miralax.
If he's started having fevers, it could be that his body is reacting to one of the treatments or foods, or that perhaps there's an internal issue with the constipation. If the fever continues for more than 1-2 days, or spikes above 100, I would take him to the dr. or urgent care for an xray to make sure that there isn't a blockage.
If he's running a fever, but won't take the Tylenol, use the Feverall suppositories.
For the fruits, make sure that you aren't offering fruits that counteract. Apples, prunes, watermellon, cantalope loosen things up, but bananas tend to tighten things up.
Also look at your daytime routine for counter-efforts. We were offering our girl all of this 'go' stuff, but then she was having a banana with breakfast and a banana as her bedtime snack. Once we changed her bedtime snack and started the Miralax, she's been doing ALOT better and going at least once a day. Yesterday she said 'Oh Mommy, it doesn't hurt anymore', and gave me the biggest hug (once we washed up!)
Finally, keep a written record of everything he's eaten, medicines given, etc. This will be helpful in not only keeping track of what's what, but helping the doctor see exactly what you've tried instead of just what you remember and help make any needed adjustments.
Good luck
M.