Actually mousemole, size IS an indicator of readiness for weaning. The new guidelines on weaning at 6m were only introduced in 2006, and many health professionals privately disagree with it. If he's 18lb at 4 months I think your instinct is right - you do need to wean him; as you say he's too big for milk to satisfy him. My DS was 17lb at 5 months but I waiting until nearly 6 months - and paid the price in sleepless nights and constant night feeding as the poor child was starving.
I haven't had similar problems to yours but firstly I would ask whether he's truly lactose intolerant, or whether he has a milk allergy. The two are very different - lactose int is an inability to process the milk sugars, whereas with milk allergy the problem is that the body perceives milk protein (casein) as an intruder, and mounts an immune response to it (see here for more info: food.gov.uk/eatwell/healthissues/foodintolerance/foodintolerancetypes/milkallergy/
How was your DS diagnosed - was he tested? If not I would go back to your GP and absolutely insist that he be tested. My GP was doubtful that my son was allergic but I insisted - turned out I was right. You could try cutting out all dairy from his diet, including dairy in your diet if you are BFing. Give him soy formula (Wysoy is what I use) - not goat or any other animal formula as it will have a similar, though milder effect to cows' milk. If after 3 weeks there's no change you can eliminate milk allergy as a cause. Bear in mind that not all health pro's are aware of milk allergy - they often assume that if there's a problem with milk it's lactose intolerance. Honestly, if we can be bothered to do the research, why can't they?
Apart from that I would gently introduce solids - one every couple of days. Try avoiding the following until 9 months:
Wheat (includes cous cous and bulghar wheat)
'Nightshade' vegetables: aubergine, peppers, tomato
Meat and chicken
Eggs
Only reason for this is that sometimes they can cause allergies. Don't worry too much if he has a little bit from time to time before he's 9 months, chances are it won't harm him.
Finally, weaning can take a few months to settle as his body is getting used to handling food.
I hope this is of some use and that things settle for you!
xx