Boys do develop later on this than girls. You sound frustrated, and I am not sure if you are letting your frustration show in his training?
You are controlling the process by taking things away instead of letting nature take it's course. However, vs. using pull-ups during the day... I would recommend trying underwear. Yes, it's messy but sometimes they do not want to get them dirty and they don't like the feeling. (This is best to wait for warmer weather as you DO HAVE TIME STILL).
Be calm, understanding and patient with him, try setting up a reward for him that he can pick. (this worked for my son, but he did not attain it until summertime. He too, is a summer birthday.).
Otherwise ~ let it go for a while. This is something you cannot force. It is his body, not yours.
EDIT:
Whoa~ I would NOT recommend an eneima unless a child is constipated and has been for a number of days and the doctor requires it. This is NOT a good practice to fall into as this can cause serious complications in itself.
As a parent that also had a 'late bloomer' with a boy that did not fully train until late summer before starting school, it is not uncommon for this to occur. Talk with your pediatrician with any concerns, but do not resort to an eneima or any thing else to make your child have a bm.
If you want to help them eliminate if they are constipated, I can help you privately by giving instructions with a tummy massage~ just email me (too many can do this incorrectly and block the child more.). My son loves a tummy massage when his belly hurts, and it helps him to use the potty easier.
I'd recommend a book called "Mommy I have to use the Potty" (I'm pretty sure that's the title.)... I have it here and I'm planning to donate it to my local library. It helped me to understand what a child goes thru during training. You have to get down on THEIR level and in the child's MIND SET, not yours...