Use lamp oil or karosene. The stuff thT goes in the old fashion oil lamps that burn. I know some folks here will made an argument about it, but over the years trumpet players have made their own valve oil from such. I have one Bach Strad that ran on karosene/ lamp oil for several years (lamp oil is karosene with some coloring and " smell good" ingredients added.)
If you have tried anything else you may need to pull the valve out, rinse well and cloth dry the valve casing, and rinse well the valve ( don't get upper corks and felt wet.
Good job mom on getting a great horn for your son.