It's totally up to you, he's your LO
. Between 4-6 months time is when most LOs will typically be ready, and although WHO suggests to wait until 6 months, in practice some LOs will be ready earlier. As you already suggested it's important to look for signs of readiness e.g. Being able to sit unaided, loss of tongue-thrust reflex and make sure if you do start before six months it is with appropriate foods.
That said..........solids are rarely the magic cure for sleep issues that most grandparents would have you believe
. Milk is by far more calorific than any 'first' solids so although solids might make him feel fuller, paradoxically by pushing low-calorie solids you can actually lead to him waking more for milk.
Personally speaking, rather than diving into solids I would first check his milk intake in the day (are you ebf? - it's possible if so that he could be snacking a bit if he's still feeding 3 hourly or more often at this age). The other thing which I would really think about if he is feeding all night long is whether you've actually got a sleep association/prop issue going on rather than a hunger issue. Does he feed to sleep at BT and every time he wakes at night? That's totally fine if you're ok with it, but it's worth bearing in mind that could actually be the cause of the poor sleeping. If you do want any help along those lines please feel free to post over on the sleep boards