Unlikely. Federal prosecutions do not work the same way as state cases. In the state system, there is often more room to negotiate back and forth over time. The federal system is much more transactional. Once the government extends an offer, that offer typically reflects what they believe they can prove and what they are willing to accept. They do not usually sweeten it just because time passes.
After 11 months of pretrial incarceration, the government has made its calculation. If the offer is 10 to 21 months and your son has already served nearly that much in custody, the credit for time served becomes a significant factor in evaluating the deal. He may be close to or within the range of the offer already, which changes the calculus considerably.
Assault on a federal officer is a serious charge, particularly in Indian Country jurisdiction where federal law applies directly. The government typically prosecutes these aggressively and trials in federal court have a very high conviction rate. That context is worth factoring in when evaluating whether to wait for a better offer that may not come.
His federal public defender or private attorney needs to be giving him a realistic assessment of the trial risk versus the plea offer. That is the conversation that matters most right now.