Doesn't make sense

Hi, I am the mother of a four year nine month old little boy. He has developmental delays including speech and language, and cognitive. He also struggles with fine motor skills as well. I can't ask this question without telling you a little about his history. His first year of life he never babbled or cooed. He never smiled. He sat, crawled and walked a few months later than you would expect. He was EXTREMELY clumsy. He would crawl/walk into things, and fall down alot. Finally at about 15 months he started smiling, but it was just with the left side of his mouth. The right side would stay closed and droop downward. Same with his eyes. the left one was more open and the right one appeared smaller and droopier. At his third birthday he was still unable to say mama or daddy. He couldn't even blow out the candles on his birthday cake. Since his third birthday though, he has started improving by leaps and bounds. His vocabulary has grown and he is putting short sentences together. His motor skills are improving and he isn't near as clumsy. His smile is even now and so are his eyes. We can't see a difference in his face at all now. He recently had an MRI and it came back as abnormal. There was an unusual white matter pattern they said. The developmental pediatrician told us that she thought it was one of the white matter diseases and ordered blood work. The pediatric neurologist said that he felt like it was scarring from a previous brain injury suffered during pregnancy or birth. I did have a difficult delivery and his heart rate dropped and then actually stopped several times. However, his apgar scores were good and he didn't need any special care after delivery. He said that the MRI's he has seen of children with a white matter disease usually had far greater areas of white matter abnormality. Everything we have read about white matter disease is that it's degenerative and the children start going down hill, but our little boy has done nothing but improve in every area, thank the Lord. This isn't making any sense to us and we are so terrified about white matter diseases. What is your opinion? I would appreciate any info or direction that you can give me. Thanks so much for your time.