I have been taking methotrexate for over 3 years.  Started out with the tabs but have recently moved to injections.  Found it so much better.  I was getting quite sick on tabs but find the nausea has really eased since moving over to the injections. 

My daughter takes the Methotrexate in liquid form but puts it in a glass of water. The day before and the day after we increase her folic acid in her diet to help with nausea and GI upset. We just started the use of injectable as an alternative. Depending on how often your son will have to take the medication and how old he is may be your best lead. There is injection site pain, but that is short lived compared to the GI upset that accompanies the pill form and also liquid form added with additional fluid. Good luck!