Hi OP.
Firstly no it's not possible that your breast milk could be poor quality. Breast milk is breast milk and there is no change in "quality" - no matter what you eat, drink, sleep, stress etc. You should try to eat and drink of course but that's for you - not for your baby.
It's odd that you have been advised to stay on the same breast for a longer time than the baby wants. This is called block nursing and is usually advice which is intended to lower supply in the case of oversupply. It's normally better to feed until baby comes off or gets fussy, wind, and offer the other side. When they come off or get fussy again, wind, back to side 1 again. You don't need to worry about foremilk and hindmilk, as this is old advice. It might be worth asking why they recommend this. If they can give you a good reason then it might be that they have a particular point for this advice, but on the other hand unfortunately sometimes HCPs have outdated information. Or perhaps they meant they want you to be feeding for at least 20-30 minutes overall, rather than just from one side?
You can read this article for more information:
www.analyticalarmadillo.co.uk/2010/07/foremilkhindmilk-and-lot-of-confusion.html
The advice to feed at least every 2-3 hours is sound, though. When babies are reluctant feeders or losing weight it can be important to prompt them if they are going too long between feeds. Of course if the baby wants feeding more quickly it's fine (and a good idea) to offer the breast again even if it hasn't been 2 hours. Frequent feeds also help increase supply.
What it sounds like to me is ineffective milk transfer. The reasons for that are not always clear cut. It could be latch issues (a latch which "looks fine" is not always fine) or physiological issues with the baby such as tongue tie, or sleepiness from a difficult birth. If you have the opportunity (and I realise it might not be possible with a toddler) it would be really useful to take yourself and the newborn off to bed or into a warm bath and just lie there skin to skin, for as long as possible, to encourage lots of feeding. Using a wrap sling under your clothing (baby in a nappy) with him in a position he can reach the nipple would also be beneficial, but again, not always practical.
You do need proper real life support to sort this out unfortunately - it's not something you can really do over the internet. If you're still under midwife care can you ask if they have an infant feeding specialist? Could you afford to hire an IBCLC? Ask them specifically for support with increasing milk transfer. Yes of course mixed feeding is important so that your DS gets enough food right now. But it would be beneficial to you both if you can get feeding sorted.
Here's another Armadillo article about how to check milk supply and things you can try (for some reason it's only visible through the <a class="break-all" href="https://web.archive.org/web/20110618111601/www.analyticalarmadillo.co.uk/2010/09/is-my-breastfed-baby-getting-enough.html" rel="nofollow" target="_blank">Wayback Machine)
The missing "breast compression" video - you can literally just place pressure on the breast with your hand while he's latched on in order to increase milk flow.
Whether or not it's okay to give 2-3 medium feeds rather than a small one after each breastfeed, I want to say yes this is better, but I'm afraid I don't actually know with a baby that young. Something you might want to look into is an SNS - Supplemental Nursing System. This allows baby to receive formula while latching from the breast which can be helpful in terms of encouraging more direct BF.
Good luck with it - I really hope things work out for you.