Hi OP,
You're definitely not failing him 
If you have stopped breastfeeding and have been giving him formula, this could have affected your supply, hence why you might not be able to express much at the moment.
To try and get your supply back up -
Buy and start taking some fenugreek, it has really helped my supply with both DC.
I have the medela swing, and I think it's brilliant. It simulates the sucking of an infant, which will help send the 'make milk' signal to your body.
Put baby to breast as much as possible. Cancel visitors and distractions for the next week, snuggle down with your baby on a comfy chair/your bed, lots of skin to skin time. 'Tummy to mummy, nose to nipple' helps too.
Look on YouTube for correct latch techniques and breastfeeding positions. Try different positions until your baby is latched on correctly. Keep calm and keep trying - this can take a while to get right. Skin to skin time helps with keeping calm for you and baby.
Appreciate that it will likely be difficult, at the start, and that's ok. For example, I have inverted nipples, which made it difficult for my DC to latch initially. I used a nipple sucker to draw the nipple out, then nipple shields with one to help with the latch, and would slip them off once they had settled into the feed. I went through a stage of my let down being too fast, so would have to stop and express a little before popping baby back on. It took time but I got there!
Babies will often fall asleep on the breast when establishing breastfeeding - it is hard work for them, plus your breast milk releases 'sleepy' hormones which helps with sleep and bonding. Tickle his feet, back, to help stimulate him. Change his nappy too before you switch sides, that usually helps to wake them up a bit!
Get support from local breastfeeding groups and your HV, they will likely have lots more advice and support to help you.
Check out these websites in the meantime -
kellymom.com/category/bf/
www.todaysparent.com/baby/10-tips-for-going-back-to-breastfeeding/
www.laleche.org.uk
You will still need to supplement breastfeeding with formula, best to speak to your health visitor about making the transition and reducing amounts. Look out for the amount of wet and dirty nappies as an indication as to how much milk your DS may be getting.
Remember, although they have an instinct for it, babies have to learn to breastfeed efficiently. Give it time. Please don't feel you are a failure, whatever the outcome. Your son is being fed, loved and looked after by you. You're doing an amazing job. Good luck 