Wow a whole 1lb a week, now that impresses me because the hospital staff would party if Elly managed 1oz in a week. If Oscar is simply following the 0.4th it may well be he is just a small child.
The 0.4th and the 99.6 centiles act as a flag for health visitors to be more vigilent as some babies, like Elly, may be failing to thrive or have other not spotted yet health needs. You may end up with a hospital referral for a health investigation just to be on the safe side. The ordinary A5 size centile charts tend not detailed enough for tracking small gains. Elly had a special chart from BCH that was designed for babies with faltering growth and or weight gain. It was A2 in size and could show very fine detail and mere hints of improvement as opposed to her standard chart. That one just looked awful for months.
The only thing I can think that might have contributed was my previous pregnancy. I had severe hydronephritis had a c/sec aas soon as I reached 37 weeks and became pregnant 8 months later. My other daughter had asymetrical IUGR but it was mild [10th centile] and she soon caught up [3months] She has moderate dyslexia and is still a dainty adult size 8. With Elly I was monitored throughout as a high risk mum and had hospital based consultant only care throughout. There is not always a reason why some babies are born at the lower end of the centile charts just as there is not always for those born at the upper end. Some babies are tiny and some are very large with the huge majority of babies falling between.The charts exist to help HV to try and pick up babies who may be quietly struggling and need a follow up to ensure they are well and thriving.
Remember my daughter had a rough neonatal period and very severe failure to thrive. Oscar is gently growing so is unlikely to have the same kind of problems. Developmentally Elly spent most her time in a baby-sling next to my skin because she was so small and struggled to maintain her core body temprature. She interacted with me and did show age aprobraite interests. It took her longer to pull herself into a stand as she was simply too small to manage to grab things to pull herself up with. she could sit but would fall over a lot but that was a gross motor difficulty] She could hold up her head and would reach for toys. She showed more interest in very high contrast toys. Her tiny hands made holding toys difficult, she went for toys with fabric tags they seemed to be easier for tiny fingers to grab hold of. She was very hypertactile and needed something soft and cuddly to stroke and hold at all times [dyspraxia]She could talk but rarely did but that was thanks to her habit of letting her elder sister do the talking for her. Socially she was smart as an example she had worked out by 18 months she could scrounge cream cakes off older ladies in the tea shop by staggering towards them and lisping 'ake'most of whom mistook her for a very advanced eight month old baby and she always got cake! She could walk but did not fit even the first crawling baby soft shoes till she was 20 months old. I bought a decent child back carrier that she easily fitted it till she was at school so she was above every-ones feet and could see what was happening around her.
Every baby develops different skills at a differing rate, as an example Elly talked early as she spent so long face to face with me she talked well and had a good vocabulary but she was slightly delayed in walking due to her size. She had good fine motor skills but poor gross motor skills. Go with your gut instinct- mine was telling me her inability to sequence, extreme continuing clumsiness and lack of awareness of where her arms and legs were needed looking at as it was not quite right even allowing for her size. Remember most babies with IUGR do fine once they begin to grow after the first few days. Elly had continued problems with maintaining her core body temp, her mouth was too small for a prem teat so she was syringe feed like a kitten. She flatlines and fell off the centile chart as she did not grow at all till she was past 3 months and she lost 40% of her birth weight contributing to her more complex specific learning difficulties. She is very bright [ed psych assessed her at 130] its just the bits controlling gross movement and sequencing probably did not get enough nutrients to help them grow at the right kind of times. Her growth was minimal till she was 14.
Your lovely son Oscar is slowly growing and gaining weight albeit on the 0.4th.so is far less likely to have the same kind of problems. Most little boys even with symetrical IUGR catch up between the ages of 2-3 years.
If you are worried you might fine it useful to contact the Child Growth Foundation in the UK as they offer support and info for IUGR.