I also agree that the solids could be having a negative effect on your LO's sleep, and that at 5 months he may not be ready to go 12 hours without a feed. Now before you feel I am bashing your choices, I am not. I also started solids early, I feel strongly though that that should be done under a doctors supervision. This is not to imply that you don't know your child, not at all, but at this age most LO's tummies are not ready. That said there are of course reasons to start. Ours was a failure to thrive.
What I don't like is that many new parents are told "start solids, they'll sleep better" Its NOT TRUE!! if you feel your reasons are valid then by all mean continue, but perhaps you should also take the time to read the information about why itsa better to wait. Old research suggest 4 months, new says 6, either way as the parent it falls to you to make the choice. When you start solids LO's take less breastmilk/formual and therefor don't get the calories they need. Breastmilk and formula are the main food for the first year of life.
As for sleeping through the night, 6-8 hours is considered sleeping through the night. The majority of babies don't sleep 12 hours without a feed, especially breastfed babies, untill after a year. Its not unreasonable to expect one waking a night. That said if you deside to wean your child and they show they are ready then of course you want to do that. go slow and be flexable. My DD for example would need a feed some nights, and nothing other nights. Each baby is different, age and weight have little if anything to do with sleeping through the night.
Also I would like to add that I see that you are reading your child well and doing great with him. Please don't take our coments as insults because they are not, they are advice and information we can share, and then you can use whatever way works for your LO.