Adoption isn't going to work here for multiple reasons. Age is just one disqualification here.
There's really 2 possible paths for him to immigrate in the "near" future from a technical standpoint:
1) She enters on the CR-1 visa and immediately files for him. If he is under 21 still he may be able to qualify under F2A. ETA: ~2 years currently (and certainly may change by that time). If he ages out into F2B, expect closer to 5-7 years.
2) Divorce (it must be finalized and both be free to marry) and do a K-1 visa. He qualifies as a child still so long as the visa is issued before he turns 21.*
*this a very highly irregular path, let alone all the disadvantages of K-1 and any possible questions that may arise about the relationship (although personally I'm not as worried about that...the circumstances can be explained).
The main issue here is a quick google search implies the timeline for a divorce in India would likely mean he turns 21 before the K-2 visa would be issued. If so....or if that is even a reasonable possibility, then this option is moot.