Please don't give him orange juice. He is very unlikely to be constipated if you are exclusively breastfeeding. It's just that breastmilk is very efficiently digested a lot of the time, especially by six weeks - and there's not much waste.
He may look uncomfortable and strain - it's because they're not really used to the sensation of pooing and it can feel peculiar. DS used to strain and strain and produce the softest most explosive nappies - they got less frequent as he got older. Some breastfed babies poo every time they feed for weeks, some begin to go less frequently and anything from every feed to up to a week-ten days is normal.
He may have been going to poo anyway when you gave him the orange juice. If it's soft/runny, don't worry.
How often do you feed him? Can you try lots of skin to skin contact and feeding him every time he opens his mouth for a day or so?
Expressing will give you absolutely no idea how much milk you're producing, or whether it's foremilk or hindmilk. Breastmilk doesn't go from being foremilk to hindmilk immediately - it gets progressively fattier as the feed goes on. But plenty of women successfully breastfeed their babies without ever being able to express anything much, so stop stressing about doing it.
And don't get your baby weighed once a week - once a fortnight is plenty - as long as he's wetting plenty of nappies and seems alert and happy. Look at the baby, not the scales. Weight gain often tails off a bit as they get bigger.
DS used to lose oz here and there too. Think how much an ounce actually is and if he'd done a big wee just before he was weighed this time, but not last time, that could've made the difference - hence not weighing so frequently.
HTH a bit xxx