It’s not parenting, some babies are just harder work. I was very smug with my DS, who would obligingly fall fast asleep as soon as he was put in his baby box when I decided it was nap time- even, once, in the middle of a very loud wake. However, he transformed into an absolute sleep terror at around 2 years old and is still, frankly, a pain in the arse with going to bed at six years old.
Some of my friends had really challenging babies who screamed and resisted sleep, up every hour etc. But most of those children had nailed bedtime by the time they were toddlers, I think I’m the only one still dealing with bedtime issues 😆
My point is that it’s a rare child who is easy going all the way through, most will be highly challenging at some point. It’s just harder when you’ve got a non-sleeping baby because there’s such a limit to what you can do, it’s not like you can reason with them!
Ignore your mum, she’s being extremely unhelpful. Perhaps you could suggest that she tries settling the baby for a nap (with you there to ensure she doesn’t just leave him to cry for ages) so she can see what it’s like.
Just remember that it will pass, it does feel like ages when you’re in the thick of it, but it will almost certainly get better faster than you think.
Just a small bit of advice (I expect you’re probably sick of people giving unsolicited advice on the subject), but do you swaddle? DS barely slept for the first few days of life until a midwife swaddled him and he was out like a light. We got very thin, breathable cotton swaddles that have a sort of bag section that you insert the bottom half of the baby into and then fabric ‘arms’ that wrap around the baby’s arms and secure with Velcro. DS was swaddled for every sleep and I do think that made a difference, he would startle awake without the swaddle. I would never have thought of it if the midwife hadn’t swaddled him in hospital, so I thought I’d mention it!