I have a similar experience with my little one, she is 11 weeks old. She has been on lactose free milk for almost 3 weeks now and the difference is unbelievable. If James has lactose intolerance the dr wouldnt think twice about prescribing it. Has it actually been confirmed by the hospital or is it something you think he has with you being a nurse. Low lactose milk is no good for a lactose intolerant baby and im suprised that you havent mentioned Colief. Colief is the first thing they prescribe before lactose free milk as it is the first line of testing. if the baby improves with colief then they consider testing. What is his poo like, they will want a sample from him before confirming anything. If James is lactose intolerant then the paediatricians will want you to hold of weaning till around 6 months and a dietician will get involved to ensure that the correct food is given and that the baby is getting enough nutrients.
I would discuss weaning with your HV or dr personally as it differs for each baby.