It sounds to me like his calorie requirements are increasing (normal at this age) - really there are different ways to handle it and it is up to you what you feel is best. WHO advice is EBF for the first 6 months, however I would caveat that by saying that is advice designed to cover all countries and circumstances and in the developed world we largely have access to clean, safe, nutritious foods which may be introduced earlier. In fact several places use 4-6 months as a guide, and some are preferring the slightly earlier introduction as there is some evidence that delaying things too long actually increases allergy risk. He's over 17 weeks which I'd consider a minimum for solids, and has some other developmental signs which suggest he may be ready. HOWEVER - he is getting adequate calorie intake from milk, albeit more often, so there is no great rush if you would prefer to wait. The other thing to consider is that the solids you introduce earlier on are often quite low in calories compared with milk. They add bulk, and add to the feeling of fullness, but may paradoxically lead to more wakings for milk to make up the calories if he feels too full in the day.
I started my DS at 5 months, he was barely making 3h between milk feeds and waking 2-3 times at night and was just grumpy and HUNGRY. It made a big difference for him, not saying it would for yours though and I wouldn't want to sway you one way or the other. Follow your instincts and I'm sure you won't go too far wrong x