Does anyone have a milk reintroduction success story

[Scrumpyjacks]

DS is 21 months and has been dairy free since we saw a dietician at a year old and had symptoms since birth. He was sick multiple times every day up until his formula was switched by a dietician. His symptoms have since disappeared.
The dietician told us to follow the milk ladder but we can't seem to get past step 1. The sickness just returns. Then the other day dh left a packet of mini cheddars within reach and DS ate one and was absolutely fine. I've therefore just given him a crumb of cheese to see how he goes.
I would love for DS to be able to have dairy! But every time we start the 'malted milk' step he is just sick.
It feels so far out of reach at the moment. Poor DS accidentally ate chocolate at Christmas and was so sick bless him.
Please can people tell me success story's of reintroducing milk and how long it took you?

Thanks

[Scrumpyjacks]

Just to clarify, I have on this occasion skipped the malted milk step due to the cheddar being OK and malted milk making him sick. I'm wondering if there is something else in the biscuit making him poorly.
I would not recommend anyone else skip steps of the milk ladder!