My stepson (Jamaican) was refused to travel in transit through the UK to Germany. He holds a visa to Germany and a US visa. Can you tell me if that was correct?
According to the https://www.gov.uk/check-uk-visa/y/jamaica/transit/no a Jamaican national normally would need an airport transit visa unless [you]
If none of the above scenarios apply to him, then the refusal was the correct course of action.
Jamaican citizens do need a visa to transit in the UK, except if they qualify for some exemption. The confusing thing is that US and German (Schengen) visitors visas are treated differently for this purpose.
Holders of regular US visitors visa qualify for an exemption when flying to and from the US but not between two other destinations. There are similar provisions for visitors to Canada, Australia and New Zealand but not for short-stay German (Schengen) visa holders so even though your stepson was flying to Germany with proper documentation that's not enough to qualify.
On the other hand, if your stepson is a permanent resident in the US or a resident in Germany then he should be able to transit without visa, even to travel between two other countries. This exemption from the direct airside transit visa requirement only applies to residents (i.e. holders of a long-stay visa or a residence permit) but not to visitors.