It's normal but you do want to help him get over this hump. It can be frustrating for a child. It's one thing if they don't understand him at school but when the parents have trouble, that's not a good sign...
Listen carefully to hear where the problem is. Is he missing vocabulary in English? for example. That's easy to fix. Make sure he knows what he's asking for. Just supply the words as he needs them. Try to insert little vocabulary building skills into your daily conversations. Make up games where he has to name things, make him do numbers and colors in the car, etc.
Perhaps he should be encouraged to stick to one language and if missing words, point or gesture what he needs. He also needs clear boundaries. My kids know with whom and when to use each language.
I had my son in a daycare where the poor women ran around like chickens without heads trying to figure out what he was saying. Don't let anyone make this mistake. Tell everyone in his life, have him show you and tell him what it is for next time.
For the record, I let the school teach my children to read and then they almost automatically picked up English. He should be working on his spoken skills at this point but if he's really keen to learn to read in English, I don't see why not. Be sure though that you're doing it correctly and not reinforcing bad habits. This was part of the reason I didn't do it myself (especially since I'm slightly dyslexic).
My children are trilingual and learning to read all three was no big deal. Their real enemy is vocabulary. It's just tough to get all those words into them. My kids refuse to mix and I taught them to "talk around" whatever they need, without dipping into their stronger language (French). Sometimes they overdo it, absolutely refusing to say it in French until I tell them to!
Try not to overcorrect. My dh does this with German and now the kids are shy to speak German in front of us, although they continue to do well in school (they're in a bilingual French-German program).